The new Portuguese coach confirmed that the signing of French midfielder Youssouf Fofana from AS Monaco is nearly complete, with no other recruitment planned.
Milan have signed striker Alvaro Morata, and defenders Emerson Royal and Strahinja Pavlovic.
"After his (Fofana) arrival, our incomings will be completed, we will need to focus on outgoings," Fonseca told reporters on Friday.
"Some players must leave now. We have many, and it’s not easy to work with a lot of players."
"We work and play to aim for the top, we cannot say that we are ahead of everyone to win the league but we want to be fighting for the Scudetto," he added.
"Inter are reigning champions, we know their level. Juventus, Napoli and Atalanta, apart from ourselves, also have the right things to compete."
Fonseca has only been Milan coach for two months but is pleased with how his players have adapted to his methods.
"I came to Milan at the right moment in my career. The feeling is very positive, right from the first day, we have already improved more than I expected," he said.
"We are expecting a different and more complex game than those we have played in pre-season. We want to show our identity from the very first game."