Jack Ross labelled referee David Coote’s performance as ‘indifferent’ but he had no arguments over the red card for Bryan Oviedo.
He is facing a three game ban and it deepens Ross’ problems at left-back with Denver Hume ruled out for six weeks with a knee injury.
With Sunderland leading 1-0, the left-back was given a straight red in the 68th minute for a reckless kick out at Marcus Maddison.
It changed the course of the game with promotion rivals Peterborough United equalising six minutes later through Joe Ward.
Jerome Sinclair regained the lead for Sunderland but Posh hit back through Ivan Toney six minutes from time.
Ross said: “I don’t have an issue with the sending off, I have watched it again. I don’t think the contact reflected the reaction, but that’s what opposition players do.
“It is out of character, he is disappointed in the changing room.
“Denver will be six weeks. Bryan suspended now. We went from a 72 hours period to having the strongest squad we have had all season to having the stretched squad. What these circumstances do is offer up an opportunity.”
Referee Coote’s performance drew regular boos and chants of ‘you don’t know what you’re doing’ from the home support.
One of the few decisions he got correct was the red card.
When asked about the referee’s performance, Ross added: “Indifferent, that happens. Strange because I hear some of the comments saying we get favours here, we have had a couple of penalties against us, red cards, we haven’t had too much go for us here.
“Our performance was good regardless of that.”