The actual shape was an asymmetric 4231. However, for the purposes of fm I think you are best setting a defensive 433 formation with 2 dms and a cm and then using the player roles and instructions to get the team playing how united did that season. Asymmetric formations are hard to create on fm, I only really got it working wonders on fm 09. For now i think its easier putting the players in a basic formation then instructing them to drop deep/ make runs etc which would essentially give you the asymmetric shape they played with. Hope that makes sense.
It's the same way managers write on paper they are playing a 442 but due to the instructions they give their players it often looks in-game like they are playing a 4411 or 4231.
The aim here is to have a balanced mentality throughout the defensive line and the 2 holding midfielders, the 3rd, more attacking midfielder had a slightly more fluid mentality and the top 3 were playing an incredibly fluid game. You definitely want to be playing and attacking strategy and tick counter attack as the team utilized that too. Again the back 6 played a more short passing game whilst the top 3 were much more direct. The front 3 closed down heavily but once the ball passed them the team stood off the tackle and allowed pressure before winning the ball back outside the box and getting the ball back quickly to the more attacking cm who launched a fast counter attack with the front three (The best example of this can be seen against arsenal in the champions league semi final 2nd leg).
I have a bit more information on this setup, but here's just something to kickstart the discussion. I'd be interested to see TBH's thoughts on this actually.