Milan began the league campaign without a win in their opening three matches but have since turned things around with three successive victories, although they have lost both Champions League matches.
Fonseca looks set to stick with the side which won the derby with Inter Milan and also beat Lecce last time out, meaning a return to the starting lineup for Spanish striker Alvaro Morata, who began the midweek loss to Bayer Leverkusen on the bench.
"Continuity is important right now," Fonseca told reporters ahead of their game at Fiorentina on Sunday.
"If the squad is in good form, it's important to keep as many players as possible in the 11 starters. That's not because I don't have faith in the others, but right now I believe it is important to have a base.
"Morata is fine, he will play tomorrow."
Fonseca's stance means Strahinja Pavlovic will remain on the bench, despite the Serbian defender impressing when he played the full 90 minutes in the three league games prior to the Inter game.
"It is important to have stability mainly in defence. (Matteo) Gabbia and (Fikayo) Tomori have played some great games and I think Pavlovic must continue to work, because he will have opportunities," Fonseca said.
"The two central players Gabbia and Tomori are doing well and I think it's important to continue like this."
Milan were outplayed in the opening half against Leverkusen in Tuesday's 1-0 Champions League defeat but did improve after conceding early in the second half. Despite the positive league run, the manager still wants a big improvement.
"Leverkusen are one of the strongest teams in Europe. We are in a period of growth," Fonseca said.
"Winning against Inter and Lecce doesn't mean we have arrived, we need to grow a lot. We are improving, but we have a lot more to improve. The real Milan is not yet close, we have to do more.
"We can have the same attitude as in the second half in Germany, but maybe the way of playing will be different because Fiorentina mark man to man, so our style will be different."
