Donnarumma has started Milan's last three Serie A games after replacing ex-Real Madrid man Lopez, who is nearly 19 years his senior.
It has since been revealed that Lopez, however, has been playing through an injury and is now set for a spell on the sidelines after being diagnosed with tendonitis in his left knee.
"I have already said I rate [Lopez] highly, but lately I didn't think he was in top form," Mihajlovic told reporters.
"He has been a real man, however, playing through the pain and not looking for excuses. We will wait for him to come back.
"But I won't take anything away from Donnarumma. He is a jewel for Milan and the future of Italian football who is playing well."
Milan have won the three Serie A games Donnarumma has started and Lopez is not the only one suffering from the new competition for places, with Keisuke Honda and Luiz Adriano also restricted to the bench.
Of their struggle to earn starts, Mihajlovic added: "A player at Milan has to be happy. If he is unhappy, he is free to leave.
"Honda, if I recall, has played. No one here is indispensable. Everyone is important and part of the squad. When you are given a chance, you have to make sure you are ready. All this passes through training as well.
"In football, with injuries and suspensions, there are possibilities to play. If a player doesn't play much, he has to get angry with himself and not the coach.
"I am sorry for Luiz Adriano because [Carlos] Bacca is making the difference. I want to congratulate Luiz Adriano though because he is doing really well when he comes off the bench.
"His two goals have given us six points. I won't forget it. He's an important player, who is conducting himself well and training with a great desire."