I agree with your assessment, but I would be very concerned if United doesn't have at least one proper DM in the squad. As you say, Casemiro's legs are shot although if it made any sense to keep him as a squad player (but one on very high wages) then we could keep Casemiro on for mop up duty after our all action midfielders secure victory by the 75th minute...although I have my doubts that his ideas on transition football will ever work with the squad that he has now, even with a few reinforcements over the summer.
I wouldn't be surprised if we sign Amrabat permanently as that DM option. His loan deal is coming to an end and there seems to be no inkling that he is going back. He could be announced on July 1st as our first summer signing.