I don't expect Chelsea to be this much comfortable in San Siro. Maybe Milan weren't able to do much against Chelsea away and lost the match by a comfortable score like 3-0. However Milan are still a huge threat in the group and I don't think they would play at a similar level to their last head-to-head match. But it is still too difficult for them to beat Chelsea of course. There isn't a single match that Chelsea lost against them in the Champions League in the past.
I think this time the scenario will be like this: Milan score first but Chelsea equalize the score. Then a 1-1 draw at the end of the match.

I won't bet on this score but I'll add both teams to score to my parlay.
I don't think, Chelsea will allow Milan strikers to achieve a goal in their second leg than to increase the goals to 4-0 for the favour of Chelsea because their new coach has introduced more tactics that will help the team to maintain the victory till the end of the tournament. Milan are still lacking in the areas of defense and midfield, which will be the advantage Chelsea will use in this second leg to achieve many goals that will make their fans believe that this season will be for their favour. If Milan coach can work on their defense and midfield before the end of this tournament, it will help them to improve in other competitions.