If City gets him, it's an offense worthy of a capital punishment that Man Utd let it happen without trying everything. I think Dembele will leave for sure this summer. The firing of Tuschel is one thing, but the biggest reason is that he has issues with his ex-agent who is taking him to court and will most likely win a large compensation over the Dortmund transfer. It's expected that his next transfer will help settle the legal issues with his ex-representative and finally clear him from his employ.
If this is true, he needs to meet with Jose Mourinho yesterday and be given the #7 shirt with a tactical setup that will let him shine. Ousmane is actually not bad at tracking back and helping on defense so I think Mourinho would be a lunatic not to try anything to get this guy.
All that being said, dude is a big Barcelona fan....so there will be that chapter to deal with down the line, but whatever he produces until then will be worth it IMO.