Ole Gunnar Solskjaer masterminded Manchester United's 2-1 league win at neighbours City earlier this month but he says they will have to produce something different if they are to get the better of them again in the League Cup semi-finals.
United's superb counter-attacking display left City's Premier League title defence in tatters on Dec. 7.
The Norwegian expects Pep Guardiola's side to be a different proposition when they meet on Cup duty in January.
United powered past fourth-tier Colchester United 3-0 in the quarter-finals on Wednesday while holders City stayed on course to lift the trophy for a third straight season after overcoming third-tier Oxford United 3-1.
"You won't beat City twice in a row or three times with the same tactics, so we'll have to do something better than we did last time because I'm sure Pep will have his team fired up," Solskjaer told reporters.
"We've looked at that game and we're very pleased with the result, and parts of that game. There are things we need to do so much better ... we've got to stamp our authority more. There aren't many that can do but we have to aim for that.
"They can out-football you. So we need to get our game heads on and be ready for them again, but we've shown them as well we can cause them problems."
United host the first leg of the semi-final on Jan. 7 with the return scheduled for Jan. 28.