The Sky Blues all but booked their place in the quarter-finals after their huge win in Switzerland.
But their German midfielder feels as though there are areas in which City can make adjustments.
He told BT Sport: "The result was maybe perfect but the way we played there are still plenty of things to improve.
"It's Champions League and it's always tough - we didn't concede which was the most important thing after these goals.
"It seemed comfortable but it was tense and Basel had chances to score."