ESSENDON fears that debutant Lewis Hayes has suffered a ruptured ACL, as the Bombers count the cost of what could potentially be the fourth season-ending injury the club has suffered this year.

Meanwhile, Sydney has told AFL.com.au that defender Nick Blakey passed his initial Head Injury Assessment after a contentious incident involving Essendon captain Zach Merrett late in the game.

Hayes, who had impressed on his AFL debut in a defensive role, landed awkwardly following a marking contest late in the third quarter and required assistance from the club's medical staff to make his way from the field.

Speaking after Essendon's thrilling eight-point win, Bombers coach Brad Scott said it was "almost certain" that Hayes had ruptured his ACL in an unprecedented run of injury luck for the club.

"Unfortunately, and we'll wait to absolutely confirm, but it's almost certain that he's done an ACL as well," Scott said.

"I don't know how you handle all of this, other than to say that when you face adversity you've got to come through it. But I can't remember a time from my time in footy where it's just week after week that this has happened.

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"We've had five debutants this year and, with the way it looks at the moment, we'll have some more. We've got to really quickly move on to the silver lining, which is opportunity for the rest of the list. We think we've got some other players in the VFL that are ready as well.

"But it's just devastating for 'Hayesy', because he's worked so hard for this opportunity and he took it. I thought he looked right at home. The positive is that he'll rehab that knee, if it is what we think it is. He knows, and I've told him, that there's no doubt now. He knows he can acquit himself really well at AFL level."

Hayes' injury is potentially the fourth season-ending blow for Essendon so far this year, with Nick Bryan (knee), Sam Draper (Achilles) and Tom Edwards (knee) also set to be sidelined for the remainder of the campaign.

It could leave the Bombers with as many as four picks at the upcoming AFL Mid-Season Rookie Draft, with Scott saying the club will need to be ready to top up its depth from the state league pool.

"Our list management and recruiting team will do their work," Scott said.

"They'll be busy, because we need to scour the country and identify the best players that are currently outside of the AFL system. I'll come in right at the pointy end of that, but their workload just went up again."

Blakey also came from the field inside the game's final minutes, having been called to undergo a HIA test after he was struck in the head by Merrett while both players were contesting a loose ball.

Merrett, who is among the Brownlow Medal frontrunners this year, had looked to spike the footy forward but instead collected Blakey, who never returned after being called to complete the HIA test.

Sydney confirmed to AFL.com.au late on Saturday night that Blakey had passed the initial test.

"He got called to do it (the HIA)," Sydney coach Dean Cox said. "I think both players were just going at the ball. The timing … that's probably all I can say from watching it live. I didn't have time to look at it after that. I was trying to get the result we needed."

The incident is almost certain to attract the attention of the Match Review Officer on Sunday morning, with Scott saying he couldn't comment on his thoughts about whether his skipper could find himself in trouble.

"I'm not sure," Scott said.

"As you know, even if I did see it … I saw the contest from the far end of the coaches' box, but I didn't see the replay, so I've got no idea. Even if I did, I can't say anything, can I? It's pretty boring."

Sydney was made to rue a series of undisciplined errors throughout Saturday afternoon's eight-point defeat, with the Swans giving up a total of seven 50m penalties – including three that resulted in goals throughout the first quarter – in the loss.

Two of the first three 50m penalty goals were as a result of players not adequately standing the mark, with Cox saying it was especially frustrating given the club was recently sent a reminder of the rule via an AFL memo.

"It was discipline, but also stepping off the mark," Cox said.

"When you get told to stand, you have to stand. We need to make sure that the frustration from giving away a free kick doesn't go to the next step, which is to the umpires and then the 50m penalties from that. We need to make sure that we're as disciplined as we possibly can be. Tonight, we weren't.

"We had a memo (about the stand rule). We had a memo from the AFL that they were going to come down harder on it. We were aware of it."

Cox said that, in all of his time in footy, he couldn't remember ever being part of a side that had given up seven 50m penalties in a game as Sydney's brave late comeback from 39 points down fell just short.

"No, I haven't (seen that before)," Cox said.

"That disciplinary stuff, we need to address. It's hard enough getting the ball. You can't give them every opportunity by giving a free kick and also giving them yardage as well."