#2 Not sticking to only one philosophy
Solskjaer has played in a 4-3-3 formation in most of the games but has changed it to 4-4-2 diamond when needed. The recent win against Chelsea is a good example, as he used Mata behind the two forwards - Rashford and Lukaku.
With Martial and Lingard injured, Solskjaer lacked a bit of pace up front. Hence he played Mata in a number 10 role where the Spaniard is at his very best. Though Juan didn't provide any assist or score, he did a fantastic job on Jorginho, stopping him from playing his natural game.
In the match against Arsenal, Solskjaer used Lukaku in a wider role down the right-hand side, and the Belgian came up with two assists. Romelu has played a couple of games for Belgium as a right winger and Solskjaer didn't shy away from that option, using Lingard as a false 9.