Changing managers tends to do very little and it's my suggestion that they keep the manager for a little longer, they are yet to finish the first half of the season so I think it's too early when it comes to the talk of sacking the manager, things might change mid season.
Not really if the problem is, in fact, the tactical choices. Milan's problem is not as complicated as United in the Premier League. They need a better tactician and leaders in the dressing room to lift their mood. With United, they have to do better because the problem is too complicated and changing a manager because the bad result won't likely resolve that. They need a full rebuild. Milan has to avoid being in the same condition as United, and changing the manager or at least add an assistant to assist with the tactics should be a viable option.