The reasons for city's ruthless instincts this season are their playmaking quality near the opposition final third and the control they provide with 2 points of contact in attacking third with a 5 pronged attack from all sides.
I think to catch them also united need a second attacking playmaker in the side who is an upgrade on Mata and Lingard both as they both are hot and cold players and a club with United's stature must also have a better playmaker than those 2 in the main starting 11. We need much more than just points and consistency to catch city, we need more creativity, control and arrogance on the ball to have even better attacking cohesive team.
Mourinho's teams had playmakers like Ozil and Schneider in his previous clubs who made their attacking play more ruthless and efficient which is not the case with current United team and also the style of the team suffers because of that as the transition is much slower to what fans want to see when Lukaku drops deeper to pick up the ball instead of stretching the line and running in behind and attacking the space behind.
Nabil fekir like playmaker would we a superb addition to our team who can also play multiple roles as a no 8, 10, and on the right wing.
Also as Jose loves a 4231 at times as well he can perfectly slot into those roles as well and make our system a 4231/433 attacking system.
---------------------Lukaku------------------
------Martial------------------------Sanchez--
----------Pogba------------------- fekir
-------------Jorginho/matic---------
This front 6 together would be immense with more creativity, better control, under the high press and better in transition. And this front 6 could easily transit to 4231 Jose system and be a hybrid 4231 /433 attacking system where Sanchez drifts centrally and fekir drifts to right wing as a left-footer cutting inside which we have been lacking in big games with Matas lack of pace, while pogba drops deeper like Mourinho is playing him right now alongside matic or jorginho.
Also, fekir would come in handy as a right winger in a defensive 433 with pogba and matic and jorginho/ any other DM could play as mid 3 adding extra quality and balance in final third that we lack.
Every top team in UCL as well have attacking playmakers to have good control and creativity in final third and United also need one if we have to improve with the ball. Bayern has it with James and Thiago, Madrid with Isco Modric and Kroos, Barcelona with Messi Coutinho Iniesta, city with Silva and kdb, Juventus with Dybala, and it's time united also have these playmakers rather than just DMS or b2b players.