15 minutes means 9 minutes for the match. So u have 6 minutes to do something. But beware that u can't make changes from 4 minutes before kickoff, so u have maximum 2-3 minutes to make your changes after the 1st match else when the clock shows 4 minutes left for the second match, making changes will get banned. I will advice u to heal players first. Then chance the formation as u can do that during the match as well, though u may need to waste a few steps. The 2nd match is the more important one as it is Cup. All the best and may God give u quick fingers