Even though both clubs played quite hard in this match but Chelsea's advantage is clearly more because they are playing with 11 men but Milan are only playing with 10 men and their defender is limping.
Besides that, Chelsea got a little bit benefited from the referee due to the violation which I think make mentally Milan down from the beginning, plus Giroud's performance is not good also, he should be able to set goals after that, because he looks free without any Chelsea player around. So, with the winning, I think Chelsea has enough confidence to continue to 2nd round, They isn't good on premier league which I think they focusing only for Champion League.
The red card was giving big pressure to the milan. The penalty is fine but red card may be too much for milan. I do agree with you the mentality from players got affected after red card was happening. That's why there are so many errors happened with milan after tomori gets red card.
The fact that if these days chelsea seems find its right track to perform so well in any of its matches. Im so glad if chelsea managed to win the remaining match in the champion league against dinamo.