The Red Devils meet Roberto Mancini's men on Sunday in the Community Shield, following a busy summer of activity on both sides.
"Because of the particular facet of the club that they have the money to spend, I thought maybe they would spend more this summer and that may still happen, they may still sign a couple more players," the Scot told reporters.
"We get this almost every year when clubs round about us buy, and that's how you view the importance of winning the league these days."
However, the United boss insists his players are ready for any challenge against a side that he says reminds him of Jose Mourinho's Chelsea.
"Chelsea had this great spell of buying when Jose first went there and we accepted that challenge. It's good to accept challenges, it keeps the complacency away from your door and we carry on as best as we can."
Arrivals at Old Trafford this summer include David De Gea, Phil Jones and Ashley Young, who are all eligible to start on Sunday.