ST KILDA is still unsure when Paddy McCartin will return to action after suffering his third concussion of the season.

McCartin was injured in a marking contest with teammate Tim Membrey and Gold Coast defender Steven May midway through the first quarter of last Saturday's loss to the Suns.

Richardson said McCartin would likely be back playing in a prior era, but flagged a cautious approach for the NAB AFL No.1 Draft pick in 2014.

"All the initial tests that you would do, the internal tests, the cog state (cognitive state test) etc., have been positive," Richardson said at a club function on Friday.

WATCH: Luckless Paddy suffers another concussion

The cognitive state test examines a player who has been hit in the head and determines whether or not they are concussed.

"Some of the external tests, with neurologists etc., are ongoing, but certainly heading in a positive direction.

"Back when I played, he'd be playing again, probably, if not this week, next week. We'll be a bit more cautious given that it's his third one."

McCartin was actually cleared to stay on the field against the Suns, but the Saints opted to take a conservative approach.

"He passed the cog state test on the day. We just didn't think it was right to get him back out there," Richardson said.

"I'm not sure when Paddy will be back. We think it'll be much sooner rather than later.

"We think he'll still play significant footy for us this year."

McCartin has been concussed five times since 2014. He ran the forward line meeting on Friday morning and was training as early as the Monday after the game.

Richardson's discussions with the doctors have not given the coach any cause for concern.

"At this stage, it's all really positive. The only reason that (the testing is) ongoing is because it's the third time," he said.

Key defender Sam Fisher suffered a reoccurrence of his hamstring injury, and will be unavailable for at least another two weeks.

"As he's starting to up the speed and up the tempo, he's just getting tight again," Richardson said.

"We need to be conservative with 'Fish', too, for different reasons. He's not a young man anymore. He's still playing really strong footy."

Fisher turns 34 on Sunday and is out-of-contract at season's end.

Another veteran, Nick Riewoldt, could be in his last year as captain. He has signed on for next season but Richardson said Riewoldt would do what is best for the club.

"Every year (since Richardson's appointment), Nick has said the same. He's come to me and said that if it's in the best interests of our team and our footy club for me to stand aside, then he's ready to do that," the coach said.

"The reality is, it's probably getting to that time. We need someone else to come through, and it'd be a great position to have Nick at least for another year, to be able to work with.

Richardson nominated David Armitage and Jarryn Geary as having performed well when they stepped into the captaincy role last year when Riewoldt was injured, while Jack Steven led the club to a 32-point victory against Carlton in round 12.

"I think (Riewoldt) would be disappointed if we hadn't identified someone for 2017 to be our skipper," Richardson said.