Once the Serie A season begins friendlies should be used for only one purpose: to keep the uncapped (and Primavera) players in top shape during international breaks. Well also to let Mancini try out new tactics or use players that haven't gotten much playing time but the point is during international breaks. Turns out somebody didn't get that memo.
Sidebar: Also, turns out there's this friendly called the Trofeo Berlusconi (not in honor of Silvio but his father Luigi) that was supposed to see Milan take on the most recent Champions League winner each year. That didn't work out so it turned into Milan playing whoever would agree to it, usually that opposing team being Juventus. The reason most Inter fans probably don't recognize this fixture is because the last time the two Milanese sides met for this, it was in 1992.
So usually this friendly complies with the general standard of trophy-related friendlies taking place over the summer, but with Milan participating in the International Champions Cup that was unable to happen. So they got Inter to agree to have it during the season. Did they decide to have it during the winter break? Of course not. Did they decide to have it during an international break? No that just makes too much sense. Milan decided to have this pointless friendly on October 21st. Not only is that during the regular season, it's 3 days after Inter take on Juventus and 3 days before they play Palermo.
For the life of me, I can't understand why the club thought this was a good idea. Not only does the team play two matches in one week already, but that next week sees Inter play twice as well (Tuesday and Saturday against Bologna and Roma). So in the space of 14 days, Inter will be playing 5 matches, two of which will be against the top two teams last season.
Whatever management's reasoning is, this match is happening. So Mancini needs to come up with some masterful squad rotating throughout those two weeks to make sure points are still picked up in the league and that nobody gets injured through overuse. If I may offer Mancini some advice, in the "Milan Derby" #playallthebabies.