But Chelsea head coach Mourinho, who has so far signed Steve Sidwell and Claudio Pizarro on frees, insists his men have nothing to fear from a United side reinforced by Anderson, Nani and Owen Hargreaves.
Mourinho said: "I'm not worried. United can only play with 11 players and if they played them all, Ryan Giggs would be out, which would make me very happy.
"When Chelsea were spending, everybody said we had the obligation to win. 'Chelsea are buying their titles,' they said.
"Now I'm waiting to see if people put us in the last place of the title candidates as they have to put United first, Liverpool second - they haven't spent much yet but they will - and then Arsenal, Tottenham, Aston Villa and Newcastle."
