Mancini has spoken about five new signings this summer.
He told the Daily Mail: "The Champions League is strange. I don't think we can start to win this yet. When I won the title at Inter, the idea was to defend the title and that should be our target this year. The same. If we also go well in the Champions League, there could also be a chance that we can arrive in the quarter, semi-final or final.
"We will try to win it, but it is important, for this, that we have a good squad. I don't think we can play for the Champions League with this squad. That is my opinion, so for this reason we have to buy."