The Scot believes that the dismissal of midfielder Marouane Fellaini in the final minute of the match for a second bookable offence was "strange".
"I thought there were some strange decisions all night, and that was another one," he told ITV Sport.
"I think throughout the night there were a few things I found odd. I think it was the totting up that cost [Fellaini] in the end."
United are currently at the top of Group A with eight points from four matches.