A SCARY collision with Carlton forward Levi Casboult in last Saturday night's clash at Etihad Stadium got Geelong defender Tom Lonergan thinking about life after football.

Lonergan, who lost his right kidney in 2006 after a collision on the football field, courageously sat under an incoming kick and was crunched by Casboult, whose knee was planted into the "good side" of his back.  

Out of contract and set to weigh up his future at the end of the season, Lonergan said retirement was among the thoughts that went through his mind after the incident.

"Nothing like that has happened to me since, so I've never really thought about life after footy until you do get a hit like that close to the kidney," Lonergan told AFL.com.au.

"You start to think, 'Well how many of these can you take? It was just one of those initial thoughts.

"It was a decent hit and I knew straight away it was on my left side, which is my good side.

"So that was the only worry, hoping it hadn't got near the kidney, and the doctors were really confident it wasn't anywhere near that."

Lonergan was taken to the rooms after the collision and underwent an ultrasound to make sure there was no damage to his remaining kidney.  

He returned to the match after getting the all-clear from medical staff.

"Sometimes you don't have any time on a footy field," Lonergan said when asked how he continues to show such courage given his circumstances.

"It was a flat kick into the forward line and obviously the player was coming in from behind.

"I was in that position hoping to take an intercept mark and that's part of the game if you're in those positions, you've got to take your whack."

Lonergan, who is in his 15th season and turned 33 in May, is out of contract after signing a one-year contract for 2017 last November.

The 2011 premiership Cat said he would discuss his future after the Cats' looming finals campaign. 

"When things like last week happen it sort of does cross your mind," the veteran defender said.  

"I think I'll have that discussion post-season. We've got a big finals campaign coming up and it's so even, so I'll wait until after the season to have those discussions."

Geelong faces Sydney on Friday night in a crunch game for the premiership contenders, with superstar forward Lance Franklin looming as Lonergan's match-up. 

The Cats will also need to overcome the loss of suspended superstar Patrick Dangerfield and Nakia Cockatoo, who suffered a hamstring injury against the Blues and joined a lengthy injury list.

Lonergan joined Essendon star Joe Daniher on Thursday morning to promote DonateLife Week, aiming to encourage Australians to join the Australian Organ Donor Register.

The AFL and its 18 clubs have partnered with the Australian Government to support the cause through round 20 and help the 1400 Australians in need of a life-saving transplant.

"This cause is personal to me. We can all contribute to saving lives this weekend by supporting DonateLife Week," Lonergan said.