Selwood was on the end of a heavy bump from St Kilda's Farren Ray as both players contested the ball on Geelong's half-back flank late in the second term.
Ray's hip collected Selwood's head, knocking the courageous Cat unconscious for as much as two minutes.
As Ray was reported for forceful contact, Selwood received oxygen and was stretchered from the ground and substituted from the game.
He was taken to hospital for scans, but was cleared of structural damage to his face and returned to the MCG post-match.
"He's got a bit of a fat lip, he's had scans so he's clear of bone damage," new Geelong coach Chris Scott said.
"He's obviously got a sore head, but his memory is pretty good. I think it takes more than that to stop Joel Selwood."
Selwood's loss left Geelong to fight for the win in the absence of four recent premiership-winning midfielders, with Ablett gone to Gold Coast and skipper Cameron Ling and Paul Chapman missing through injury.
In that star quartet's absence, ever-reliable Jimmy Bartel led the way for the Cats, racking up 27 disposals and kicking a brilliant left-foot goal from deep in the pocket early in the final term that put Geelong ahead for the first time in the match.
The Cats were also well served by veterans Joel Corey and Corey Enright, while David Wojcinski at times threatened to break the game open with his trademark pace and dash.
As expected, Steve Johnson spent time in the middle after being used almost exclusively as a forward under former coach Mark Thompson, and Scott was pleased with his output.
"We were pretty happy that Steve played the role that we asked of him," Scott said.
"It wasn't the sort of game where anyone could really dominate and kick a bag of goals or really get a lot of space, but we think Steve's got a lot to offer in the midfield.
"He's got close to elite endurance. That's something that I didn't realise as an outsider, so we'll attempt to use that as much as we can."
And happily for the thousands of Geelong fans left with a jumper bearing Ablett's old number, his replacement in the No.29, Cameron Guthrie, made an impressive debut.
The increased pace of AFL football seemed to impact on the first-round draftee's kicking, but he showed a clean pair of hands and excellent awareness.
"Cam Guthrie we thought today looked like a genuine AFL player," Scott said.
"First gamer, on the big stage, but he does the sort of things that we like in Geelong players."
