In the last 4 games Manchester United have deployed a 3 at the back system which United have not used for a while and a system Jose Mourinho has not used before. Man United used this system against Rostov home and away, Chelsea and Middlesbrough.
Jose Mourinho used this system against Rostov to counter there 3 at the back and the same was done again for Chelsea and on both occasions it suited United.
This system helped united with their attacking aspect because Valencia and young were deployed as wing backs and this gave united width, pace and crosses into the box.
Mourinho used different combinations at the back e.g. Rojo, Smalling, Bailly and Bailly, Smalling and Jones. There were times that the defence looked rocky but overall they looked very solid together.
United had Ibrahimović suspended for Chelsea and Middlesbrough and this system gave the 3 united forwards freedom to combine with one another through, Mata, Lingard and Rashford.
When Mourinho used this system for the first time many united fans consider this to be an experiment for the game against Rostov but having used it in the last 4 games fans are split if this should be a regular system or United should go back to 4.
But many fans are happy to see that Mourinho is trying different tactics and ways of keeping his team on all fighting fronts.