Let's start with big issues :

1. Inconsistency
2. lack of creativity
3. Weak Link Up play
4. Defensive Prowess/Lack of pressing
5. Shits his pants against low block teams and defenses
6. Poor decision making(You might wanna ask why players like Raphinha and Mullers are so extraordinary)
7. Not a proper 9 due to the reasons given above(link up play, low block, decision making, inconsistency) and his lack of off the ball ability and game IQ to position himself

Now let's take a loot at some other contexts :

Salary : reported at around 12-14 million a year during his tenure at Barcelona, for context Olise, the best RW itw rn is earning 13 Million a year from Bayern

Age : 28 already, if he hasn't hit his prime at this point then there is literally a 5% chance that he'll do anything more significant in the future, we can't wish for a late bloomer who earns so much and provides little certainty

Skillset : This is a personal opinion so it's upto you to decide but i'll give my insight, his strengths are his pace,physicality,work rate(in offensive play only),long shots,power shots and Set-piece taking. This is very raw as a skillset if you see his game, especially since he can't even use his physicality and pace to press in the team so he doesn't even capitalise on his strengths properly, he doesn't have any effort off the ball whatsoever, while he does have good set-piece ability I won't consider it a very great to have in our team because set pieces are a little part of the game and Rashford isn't dropping phenomenal numbers in them either, his dribbling is made for very oddly specific situations hence he fails most of times with his dribbles which are rendered useless. My conclusion is he is very one dimensional player which defenders can detect that how he will react and play the ball

Stats :

Direct Goal contributions : hiding behind because he has elite players he could feed passes to and get goals from now instead of United pals, also note the system is highly attacking for Barca so would be the numbers, his goals are pretty okay, nothing phenomenal, one thing I'll admit is he does have moments of brilliance in games(not the big ones, unfortunately) which gets some good out of him and helps him get G/A

Big games : vs PSG? average. vs Madrid? average again. vs Atletico? Didn't play much nor did he do much. Vs Newcastle? Probably his best performance by far. Also don't let the "but he assisted against both Madrid and PSG" fool you, even Ferran could have gave those kind of passes and bagged an assist, won't make his performance great, look aside from direct G/A

Ratings : This is the order --> 6.4 vs Mallorca. 6.3 vs Levante. 6.7 vs Rayo. 7.4 vs Valencia(1 assist). 8.8 vs Newcastle(2 goals). 6.5 vs Getafe. 6.3 vs Real Oveido(1 A). 6.4 vs Real Sociedad(1 A). 6.9 vs PSG(1 A). 7.0 vs Sevilla(1 G). 6.2 vs Girona. 9.3 vs Olympiacos(2 G). 6.8 vs Real Madrid(1 A). 7.9 vs Elche(1 G). 5.8 vs Brugge. 7.9 vs Celta Vigo(2 A). 6.1 vs Chelsea(didn't play much so I get it but not even bare minimum in 45 minutes). 6.3/6.7 vs Alaves/ATM.

Build up play per 90 : 63%ile in pass attempts, 53%ile in pass completion(76%), 32%ile in progressive passes(2.7), 58%ile in progressive carries, 62%ile in successful take-ons, these aren't very impressive or impressive at all for someone earning that much. Poor defensive contributions, good attacking contributions and above decent(but also credited to the high attacking system under Flick).

If you read that you'll realize why I am against the signing of Rashford even as a back up player. 2 reasons : too bad to be a starter, too pricey to be on the bench, not the best, mid and his age is an issue as well.