On purely footballing terms, we need a good number 9, that's the role he'd play, and he could do it for at least 2 years whilst Rashford develops further. So it actually makes sense, more sense than even Griezmann who isn't a number 9.
The ludicrous transfer fee and wages it would take to get him here is something for the number crunchers at Old Trafford to work out. If they can make the numbers make sense, then that's fine.
Him and Jose were trading minor insults when Jose left, so there's that too. In fact, that's probably what's most likely to scupper a move back.