Oxford boss Chris Wilder was disappointed with the red card that saw his team reduced to 10 men in the 1-0 defeat to Chesterfield.
Ritchie Humphreys scored a beautifully-timed volley to seal the points for the visitors, but Wilder was more focused on the sending off dished out to Andy Whing for a challenge on Eoin Doyle.
Wilder said: "I thought the tackle on Dave Kitson a few minutes earlier was no different, and I didn't think Andy's was a red card.
"He slid in, it was a sliding tackle, and he didn't mean to take the man.
"I thought the referee made a very quick decision on it. He should have taken a moment and then given the correct decision, which was a yellow.
"Having Andy Whing missing three games now is more important, and that's a big blow to us."
However, Wilder said he could have few complaints about the final result, and was very impressed with Chesterfield.
"I thought they were street-wise but as well as that they're the best side we've played by a million miles.
"I said in my programme notes it's one of the most talented squads that's been put together in this league for many years, and after seeing them against us, I haven't changed that opinion.
"That said, we under-performed and didn't turn up on the day, collectively and individually.
"We didn't ask enough questions of them, and we lacked a bit of belief."
Chesterfield might have added further goals in the game with U's keeper Ryan Clarke stopping a near-post effort from Tendayi Darikwa in the first half and turning aside Jay O'Shea's drive late on.
Deane Smalley drove wide from a good position for Oxford, and also headed against a post late on, but once Whing was dismissed, Oxford were always up against it as Paul Cook's team maintained their four-point lead at the top of League Two, this time from Fleetwood, who have taken over in second place.