Both will be assessed this week, with Sandilands suffering glute tightness and Mayne catching his right ankle under the leg of Cats' opponent Joel Hamling when tackled across the boundary line in the second half at Patersons Stadium.
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Mayne, who was reappointed to the club's leadership group during the week, spent some time lying on the ground in agony before being helped off the field aided by trainers.
The 24-year-old was walking in the changerooms after the incident and Lyon was positive the runner-up club champion had escaped serious injury.
Eagles claim early bragging rights
"We're pretty confident it's a ligament in the ankle and he'll be running within seven days," Lyon said.
"Obviously we need to check that off with a scan.
"It didn't look good. I felt a little sick in the box, obviously when you see one of your players get hurt.
"Aaron (Sandilands) just tightened up, a bit of a glute. We'll check that out as well.
"It's not perfect, but it doesn't seem on the surface anything too [serious]."
The injuries were the only sour point in an otherwise impressive start to the NAB Cup by the home side, who were led to the 7.6 (48) to 4.6 (30) victory by prolific midfielder Michael Barlow.
Barlow showed he could push to join the game's elite midfielders with 15 disposals, three clearances and two inside-50 entries in the shortened match.
Without a number of its stars heading into the fixture, including captain Matthew Pavlich, David Mundy and Nat Fyfe, the club was intent to use the early stage of the pre-season competition to expose some younger players.
And Lyon would have seen some encouraging signs, with midfielders Cameron Sutcliffe and Tom Sheridan covering the ground nicely, and Michael Walters adding some class close to goal.
After coming from behind to beat West Coast in the opening game of the night, the Cats started slowly against Freo, conceding four of the first five goals.
But a comeback in the second half, spearheaded by two goals to 22-year-old forward Mitch Brown, put them back in the contest before Fremantle kicked the final two goals to win comfortably.
After his match-winning supergoal against the Eagles, Cats captain Joel Selwood was the most impressive Cat, collecting seven disposals and booting two goals.
Super Selwood leads Cats past Eagles
The loss could be costly for the Cats, with star midfielder Steve Johnson likely to come under scrutiny for contact made with Fremantle's Hayden Ballantyne.
Johnson's 'love-tap' to Ballantyne's midriff did not appear to be of serious force, but did leave the Fremantle small forward hunched over for several moments.
There is history between Ballantyne and the Cats after a spiteful round one meeting last year, which saw Geelong defender Matthew Scarlett suspended for three weeks for striking him.
Ballantyne was also suspended for two weeks for striking Cats veteran Paul Chapman in the same match.
WHAT THE COACHES SAID
Fremantle coach Ross Lyon: "Our speed and intensity was evident against Geelong early and then they gathered themselves at the break and came again at us. We turned it over in our [defensive] 50 in a manner that we haven't in a long time. I think it's quite easily fixed, so that doesn't concern me too much, but it's a good reminder of where you can get into trouble in the modern game."
Geelong coach Chris Scott: "We got some quality game time into most of our list. Chapman, Mackie and Kelly were the only notables that were available that we didn't bring with us. I think that's a good thing this year compared to last."
Fantasy snapshot
It's hard to go past Joel Selwood (MID, $530,900). Selwood grabbed 48 Dream Team points against Fremantle and only had seven disposals, showing he can score even when he doesn't have the ball. Sixteen of those points came through tackles, a big part of Selwood's make-up as an AFL Fantasy option.
FREMANTLE 0.4.2 0.7.6 (48)
GEELONG 0.2.4 0.4.6 (30)
SUPERGOALS
Fremantle: Nil
Geelong: Nil
GOALS
Fremantle: Walters 2, Crozier, Mayne, Mzungu, C.Pearce, Silvagni
Geelong: Selwood 2, Brown 2
BEST
Fremantle: Barlow, Walters, C. Pearce, D. Pearce, Sutcliffe
Geelong: Selwood, Brown, Duncan, Johnson
INJURIES
Fremantle: Mayne (right ankle)
Geelong: Nil
Reports: Nil
Official crowd: 21,804 at Patersons Stadium
The Dockers have held on and run out 18 point winners over the Cats.
GOAL DOCKERSWalters marks just before the siren sounds, 15m from goal. He kicks through the ball and it goes through for a goal.
There will be no fight back from Geelong in this game, with the Dockers leading by 12, with less than a minute left in the game.
GOAL DOCKERSMzungu takes the chest mark 20m from goal directly in front, he kicks a goal to seal the game for the Dockers.
BEHIND CATSHawkins marks 40m out on the boundary, he goes for goal but he pushes it away to the left.
The Cats trail by seven points and we are starting to have a sense of de ja vu, with the Cats coming home hard.
BEHIND CATSJohnson has a quick shot on goal, but it is smothered and goes through for a point.
Mayne is in a world of hurt as he calls for a trainer, clutching his ankle. This does not look good for the Dockers.
GOAL CATSIbbotson with a clumsy high challenge on Brown, which gives him the free kick 40m from goal on a slight angle. He kicks his second goal and all of a sudden the Cats trail by eight points.
GOAL CATSDuffield chops the arms of Brown 35m out from goal, he takes the free kick and gives the Cats some hope of a fight back.
BEHIND DOCKERSPitt marks 40m from goal on a slight angle, he comes in and pushes it away to the left of the goal.
GOAL DOCKERSSilvagni gets himself into good position and takes a mark 35m from goal directly in front. He kicks a goal to give the Dockers a 19 point lead.
Ballantyne is down behind play, subsequent replays show that he got a 'how do you do?' from Johnson with a tummy punch.
BEHIND DOCKERSCaddy gets into trouble as Mzungu nails him, he is given the free kick at goal 40m from goal, he starts it left and it stays there.
BEHIND DOCKERSGriffin is taken high 30m from goal on a slight angle. He takes the free kick and he hits the post.
Both teams are testing each other out, just to see what they can get away with.
We are back under way for the second half.
The Dockers go into the break holding a 10 point lead over the Cats.
BEHIND CATSCaddy gives it off to Motlop who heads for home 55m out on the boundary and it just can't swing around enough.
BEHIND CATSBrown is held onto 35m from goal, he is awarded a free kick and he kicks it away to the left.
GOAL CATSSelwood marks on the wing, he is infringed and is awarded a 50m penalty. He kicks from inside 50 and slots through just what the Cats wanted, a goal.
BEHIND CATSCorey has a shot at goal from 55m, it falls short and through hands it goes through for a rushed behind.
BEHIND CATSDuncan launches a ball towards goal but it goes away to the left.
Caddy looks in good shape, he looks like he has bulked up from his time at the Suns.
GOAL DOCKERSWalters comes out on the lead and takes the chest mark 40m from goal on a 45 degree angle. His kick is a wonderful looking kick and he extends the Dockers lead to 20 points.
GOAL DOCKERSWalters breaks away from his opponent, he handballs it over the top to Mayne, in the goal square who makes no mistake.
GOAL DOCKERSCrozier makes the most of his possession, as he roves it, heads for goal and drills it. The Dockers are up by eight points.
BEHIND DOCKERSThe ball comes off hands and through for a rushed behind for the Dockers.
Varcoe roves the ball 30m from goal, he heads for goal and the pressure that was put on him, has him kicking it out on the full.
An interesting move by the Dockers, with Silvagni going down to play the deep forward role.
GOAL CATSSelwood is given a free kick for being dumped into the ground. He kicks from just inside 50 and he kicks the Cats first goal of the game.
GOAL DOCKERSPearce marks inside 50, he takes the set shot at goal and splits the middle.
BEHIND DOCKERSHill breaks free from the pack, lines it up for goal but he is off target.
The ball is bounced in the middle and we are under way.
The second game of the afternoon is between the Dockers and the Cats. This is promising to be a great match. Let the game begin!