If Milan are out of CL this early they won't only concentrate on Serie A but they will also have Coppa Italia as another objective. Instead of playing reserves they will play the best team.
I hate the risk of Milan making the final or winning the CL, but I'm still all for Milan making the quarters and be kept busy for at least 40-50 more days.
There is no such a thing as a confidence booster in this case. Milan are superior to the majority of teams they will now face in serie A. They will still underestimate those teams and let Zlatan do the job. Nothing will be changed in serie A, except that if Milan don't play CL they will definitely always play their best in serie A.