With Solskjær opting for a 2-man base in midfield, it leaves a lot of scoring responsibility on the front four. If we put Bruno to the side then the front three need to contibute 20 league goals a piece at the very least via non-penalty goals and assists.
United do not create with their FBs nor are their CMs going to contribute much to goals. Fernandes is a goal contributor but the onus is mostly on the front three. This season, only Martial (23) reached 20 non-penalty contributions. Rashford (18) just fell short.
If the front three can each score 15 non-penalty goals (45) and Fernandes can score 12-15 in total himself (57-60) then you still only have 60 goals. If the CBs, CM, FBs and bench can't add another 20 or so then you need to be conceding around 25 goals or less in the league.
For United to get into the 90-point range in a replicable way they need to add more non-penalty goalscoring to their starting XI. Rashford played 2,643 minutes in the league — 29.4 90s — but only managed an inefficient 11 non-penalty goals.
Greenwood scored 10 but did do so in just 1,320 minutes — 14.7 90s. If United do not acquire Sancho then I am sure Mason will improve much. His weaknesses are not inherently in the third phase but in the first and second phases. Goal contributions won't be an issue for him.
United were good defensively this season — one of the best in Europe actually — however they still conceded 36 goals in the league which is around 5-10 too many for a team that wants to win the title with 85-odd goals scored.
Overall, United are quite a bit off in terms of challenging for a title and a lot of it is because of the aforementioned. Tactics, squad depth, injuries and more can all be mentioned but United need more end productivity.
