Goalkeeper is one of most important position in a team. In important moments and matches, goalkeeper helps a team to protect leading result and makes it to final win or can make saves and keep up hope to equalize or score decisive winning goal.
That's right, the goalkeeper is one of the central positions in a team. Having a goalkeeper who is qualified and mentally strong will really help the team to navigate the season with few goals conceded. Sometimes the goalkeeper can also be a motivator for all attacking and defensive players to play better.
Donnarumma has good skills, good confidence and he is very young with a bright future and long career ahead. With the growth of Milan, and with their ambition to come back, they won't let him go. If Milan let him go, they would have done this a few seasons ago, not now.
I'm sure Donnarumma is having a good season this time compared to the previous season. This season Donnarumma has made an excellent contribution to a goalkeeper and he has been the last defensive symbol for AC Milan which has kept the team from losing so far in Serie A.
However, without a strong defense I believe the goalkeeper could not hold back attack from opposing attacker so of course more goals are scored. So far Donnarumma has conceded 12 goals (if I'm not mistaken) in Serie A, I don't think that's a small number for the team leading the table. AC Milan must improve its defensive system if it is to end the season with a title.