Lee Johnson saw Genseric Kusunga's sending-off coming after his Oldham went down to a late goal in their 2-1 defeat at MK Dons.
The Dons had to come from behind as a poor display led to them going in at half-time 1-0 down, but goals by Samir Carruthers and Ben Reeves after the break renewed their play-off hopes.
Johnson said: "In hindsight I could have taken him off, but people are professional and he's let the team down.
"Fair play to him, he stood up and apologised to the lads, but through some real dopiness, really, he's allowed to get himself sent off.
"Probably, people could see it coming and even we could, but you expect someone who has played in the Champions League to be sensible enough not to make a rash challenge like he did.
"The result is a real gut-wrencher because the boys were fantastic.
"We had some really good chances - Jon Stead's one-on-one in the first half, Gary Harkins had a volley and we had some good corners and free-kicks coming in.
"We also had Harkins having a shot when he could have slipped Stead in, so it's just gut-wrenching with the way it's all panned out in the last couple of minutes."
Oldham went ahead after 37 minutes as Harkins' chipped cross was flicked into the far corner by the head of Adam Lockwood.
A superb individual goal by Carruthers brought MK Dons level early in the second half as he rode two challenges and side-stepped another before finishing into the bottom corner.
Kusunga was sent off after being shown a second yellow card with 25 minutes left and the hosts went on to snatch the points when Mark Oxley pushed Carruthers' cross into the path of Reeves.