Arne Slot said a 'clear and obvious error' had denied his side pulling level in the first half but the Dutchman was reluctant to blame that decision for the defeat.
Virgil van Dijk thought he had equalised but saw his header chalked off due to Andy Robertson being adjudged as offside and interfering with play.
Slot said: 'He didn't interfere at all with what the goalkeeper could do. Immediately after the game someone showed me the goal that the same referee allowed – City against Wolves last season (when Bernardo Silva was on the line for a John Stones header).