The Blues midfielder, who scored his first goal for the club in the 2-1 defeat to City at St Andrew's on Saturday evening, sustained a first half groin injury during the game and is doubtful for the next home game against 'Boro on Tuesday night.
"Nicky's got a groin problem," manager Steve Bruce said after the City game.
"He had it at half time but we're hoping we've caught it in time and it's not too serious."