Milan are also playing the cups, so tiredness also plays a determining factor in my opinion
Maybe he has too old a roster of players? I do not know
I too am amazed at Milan's poor performance, it almost seems like a second Inter
Yes, fatigue can be said to be a factor in whether they will play well or not. Because fatigue will greatly affect their focus in the game, so that will determine what will happen at the end of the match. In the midst of a busy schedule, the coaching staff must be able to anticipate this by rotating players, for example. There have been a lot of teams I've seen that have had to decline because they had players who weren't as fit.
I would guess that the most common one is that cup games of most nations are played by either young players or just bench players teams can't get much out of and that causes the teams to have some rest. Not all the players, but most of the players can rest in those games and that way they end up with a good return when the games start again and they are rested.
You can even do that in some of the league games, not rest every single player, but replace 2 player each game to have a full rotated rested players in less than 2 months, every 2 months one player gets a rest, that's a good cycle and should be considered as a good way of getting rid of fatigue as well.
Of course, there are many ways to keep players in good shape. One of them is as you said earlier, they can also measure the strength of their opponents, if the opponent is a weak team it can be their chance to rest several players. They can start by resting only 2 or 3 players, and after they have a big lead, they can replace a few key players to keep them in great shape. Things like this should already be part of the plan in a team.
Cremonese vs Atalanta will be an open match in Serie A this week. Atalanta's chances of winning are clearly above the home team. Cremonese is one of the worst clubs this season by occupying the bottom of the standings with only collecting 13 points. It also indicates that they will be the first club to play in Serie B next season considering that it will be very difficult for them to stay in Serie A. This is the right time for Atalanta to win.
The match that will be eagerly awaited is the match between Napoli vs AC Milan, it will be a match that will affect AC Milan's position later. As for Napoli, the match will not affect their position much because Napoli are far from the club who are in the runner-up position now.
Lazio have found a comfortable opponent for them to maintain their current position, their opponent is Monza who are currently in 13th place in the standings. From the 5 matches that Monza has played in the last match, they only got 1 win, 2 defeats and 2 draws. So Monza shouldn't be a big problem for Lazio even though it's their home game.