David Hunt argued Deane Smalley deserved better after some Oxford United fans appeared to cheer when the striker was substituted.
The change was made on 54 minutes, shortly after Fleetwood Town had struck their second goal at the Kassam Stadium.
Smalley had not been able to cause the visitors many problems, but the same could be said for the majority of his teammates in the 2-0 defeat.
A section of the U’s support had already chanted for James Constable to come on and when it happened, some seemed to be celebrating Smalley’s withdrawal.
Hunt thought it was harsh to single the striker out.
He said: “Deane’s worked very hard for the team and wants to give his all.
“Earlier in the season he was firing on all cylinders and unfortunately he had an injury which has set him back a bit.
“I understand frustration, totally, but when you see things like that it’s not nice.”
The full back admitted there had been times where, as a spectator, he had been disappointed by a performance, but had not considered such action.
“You get frustrated in regards to having your own opinion, but I don’t think I’ve ever cheered for someone to get subbed,” he said.
“I think that’s a little bit disrespectful.”
On the game itself, there was a disappointment at the manner of Fleetwood’s goals.
The first came from a corner, which was allowed to reach David Ball at the back post, while Antoni Sarcevic buried a free-kick routine to give the Cod Army a cushion.
While accepting the overall display had fallen short, conceding from set-pieces increased the irritation for Hunt.
He said: “It’s more the fact of lads just switching off.
“It’s happened too many times recently and we should know if we’ve got a job then we make sure we do it.
“I can accept a worldy goal, I can accept freak goals, but these ones do hurt.”
He added: “Unfortunately it’s been two set plays which have decided the game, which we know we should do better with.
“But other than that it hasn’t been a good performance and is something we’re going to address.”