There are different rulings for different things. The devil logo is a metaphorical devil, not the actual representation of the devil, because nobody amongst us knows how the devil looks like. If he prays in that attire, then the prayer won't be accepted as it has a picture of some animated object.
Apart from that, I don't think there would be any issue as he is not going out there and promoting the evil that the actual devil has promoted.
Also, a lot of things depend on the intentions, so the rulings can't be straightforward on a few things like this.
I am sure he might have checked on it from an Islamic scholar before joining United, and he must have given him the clearance with explanation/ proof that he can wear the logo during games.
Even after that, if he still personally thinks it is wrong to wear that logo, he can repent and give some charity to the needy.
There is an entire process that muslims follow before making a decision when they encounter such dilemmas in life.
• Flopped at PSG ❌
• Knocked out of French Cup ❌
• Ghosted against Bayern in UCL ❌
• Lost US Open Cup final ❌
• 1 MLS goal all year ❌
• Hasn't played a match since August ❌
Result: FIFA The Best winner