Statistically in terms of "xG" if people want to look for a stat that looks at shot quality over a long period of time, we're 3rd in the league for xG scored (both overall including Mourinhos time, and if you only look at our time under Ole, main difference is we are closer to Liverpool since mid December). Our problem is our lack of control and ability to dominate games through the middle, pretty awful end product from the full backs, getting getting beat by the distinctly average defence we have.
Our attack obviously lacks a right winger to complete it, but in terms of chances we create on a regular basis, it's fine all the same. A right winger could put it up around Liverpool/City level, but the midfield and defence is sorely lacking and we cant sustain pressure like they can, cant build up like they can and can't dominate like they can. 2 top midfielders would make a world of difference, if they're the right type, as would a centerback or right back.
In terms of front 4, Martial, Rashford, Pogba and a new right winger is perfectly fine going forward. All young, all will improve to varying degrees but over a season and with the current depth we have, that's fine. The right winger needs to be complete the attack basically, and it is flawed until that player is signed. Get a better foundation behind them though and the attack can get the ball more frequently in good positions and just get more chances to show their quality because the rest are better.