#2 Putting Scott McTominay at center back in the second half
Manchester United started this match with Phil Jones and Chris Smalling at the center back positions and Jones put in a poor display in the first half and also got himself injured. Solskjaer put on Anthony Martial in place of him in the second half and told Scott McTominay to fill in at center back.
Safe to say, it wasn't a good decision as McTominay couldn't really apply himself at that position. The second goal scored by Cardiff was due to a direct error made by McTominay. From the throw-in, McTominay couldn't deal with a simple ball played at him and missed it all together which let Josh Murphy through on goal who picked out Mendez-Laing for a tap-in.
McTominay is a central midfielder and if this match was anything to go by, he shouldn't be played at center back and Solskjaer would have learned that against Cardiff.