Maybe I am being too harsh to judge him like this, but I watched him play live against a journeyman pro at Indian Wells a couple years ago and Musetti just completely quit in the match when things got tough. He showed zero effort going after shots, while also whining, complaining, and smashing his racquets. Instead of problem solving and coming up with a new strategy to try to win, he just bailed for the entire final set like a spoiled junior. I don't often see players that go on to become Slam winners that show a lack of grit and fight like Lorenzo showed that day.
Musetti is only 21, so it's too early and unfair to write him off yet. Maybe he just had a bad day and I happened to witness it. However, to answer the OP question, I believe that no coach can cause you to have motivation and fight if you don't already have it within yourself. Perhaps Lorenzo actually does have the moxy to be a top 10 or top 5 player, and he just needs the right guy to unlock it for him. If so, I'm not sure who it is that will click with him best. Who did Musetti admire growing up? Would a tough love grinder coach like Magnus Norman be the right fit? I don't know him well enough to know.