Utd do have a lot of injuries at the start of the game, nothing to do with your training/coaches. But should always hire the best coaches anyway to make your players iprove better. Try using any of the X-PER tactics they are designed around Utd and are doing very well for me

At the start I always sell Fletcher,Anderson,Young and usually buy Verratti as Utd CM is not very good then I sign Mayke from Cruzeiro he is a FB to rotate with Rafael. I usually sign another winger like Memphis Depay as Utds wingers are not that great either. I try not to sign to many 1st teamer usually 3, that way keeps the squad balance ok and helps making your tactics fluid quicker. After that I tend to sign players for my Reserves and U18s then develop them into my 1st team after a season or 2.