If De Rossi let Roma play attacking in quarter finals, it will be very risky for Roma because AC Milan are experienced enough in knock out matches and they have enough star players to punish any mistake from Roma players.
I agree their matches will be very balancing and they are challenging opponent for each other so to beat their opponent, they will need to play very excellently or need their opponent to make mistake and give them a big gift to score winning goals or perhaps mistakes from referees and VAR referees.
But if Roma don't play attacking it might be more difficult for them to eliminate Milan. Because I don't think Roma are really good at playing a defensive game. That boring style of Mourinho was one of the reasons why they didn't have a successful run in the Serie A. They should be able to make up for their defensive weaknesses with their attacking strength.
You have a point about Milan though. We saw what they did to Napoli in the Champions League last season. This is what making them the favourite of the matchup here. But we will see whether they will show another example of that.