Oldham boss Paul Dickov was surprised by the late scuffle after his side's 2-0 defeat that saw MK Dons defender Antony Kay sent off for lashing out at "good mate" Robbie Simpson.
Goals from Darren Potter and Daniel Powell handed the Dons a routine 2-0 opening-day victory at stadium:mk before the pair squared up at the final whistle.
Dickov admitted the brawl took him by surprise as Simpson and Kay - making his home debut - were the best of friends before the match.
He said: "There was an elbow on James Tarkowski about two seconds before it and tempers were flying a little bit high.
"It's a sad one because Robbie and Antony Kay are good mates. I didn't see it, but the referee has sent one off and not the other on the view of the linesman, when really I think the referee is closer.
"It was a bit surprising, but I've played against mates before as well and when you're out there it doesn't matter.
"The Dons bossed the first half and Potter stole in unmarked to head home a Dean Bowditch corner to put them ahead after 34 minutes.
Powell made it 2-0 in the 59th minute when he burst into the box following a swift counter-attack and fired a fine left-footed finish into the top corner before Kay was shown red at the death.
Dons chief Karl Robinson said: "I thought we looked very strong and very professional - we weren't as fluid as we always are but it was the first game.
"People keep telling me that it's one of the best Oldham teams they've seen for a long time so it's very pleasing for us that we can roll our sleeves up and fight like we did."