Belletti arrived from Barca for £4million after Chelsea ended their interest in Sevilla's Daniel Alves because of the inflated price tag of more than £25million.
The favourite to move on is Arsenal target Diarra. He has already indicated his desire to leave and with the success of Mikel John Obi in midfield, Mourinho may be ready to grant his wish.
Mourinho said: "First of all, this new player gives me what I need which is a closed door in my dressing room. That is very important for me.
"The market is open until August 31 but not for us. We have the new signings we fought for and we have the balance in the squad.
"But I want my squad to be just a little bit smaller. We have 27 players because Belletti comes in but Arjen Robben goes out, and if we can, we will go down to 26.
"Our squad is a very balanced one. We have two players for each position and the right-back position is a position where we have more than two.
"For the interest of the players, because everyone likes to play and not just be part of the squad, if clubs show a real interest in one of our players and our player shows interest in leaving for a better situation, that is okay.
"But it is not a drama for me to keep 27 because they are all good players, they are useful and the season is long."
