That legend term is a tricky thing. It is highly highly subjective so you can't say "he is and he isn't". You must take everything in consideration. And even that you can argue all day is he or not. For me things which i look are:
1) When you (or some neutral fan) mention player's name, is Man Utd first name which comes on your mind.
2) Was that player just quality player who we bought to help us to achieve our goals or he was much more.
3) Can that player after career say "We" for Man Utd?
4) Results in club
5) How he left our club
6) x factor
Some examples; Zlatan and RVP. Both have one excellent season for us but i treat RVP as our legend. Zlatan i don't. RVP's impact and aura around him was something special for me.
Phil Neville, Butt, Carrick and O'Shea. They were nearly whole career here and won everything. Only O'shea i rate as a legend. Don't ask me why, i didn't even liked him or rated him much.
Yorke, Teddy and Ole. Only Ole is. (Note; see, i don't hate the guy. Just don't want him as a manager.
)
Stam and Bruce. Stam was world class defender and among our best in history. Bruce was just a very good defender. But Stam is not a legend. Bruce is.
Hughes and Cole. Both scored a lot. But i don't think that Hughes is a legend. Andy Cole (of course) is.
I can't talk about older players (of course that sir Bobby, Edwards, Best... are legends) but in my era i would say;
Rooney as no1. Others would be Rio, Vida, big Pete, Gary Neville, Scholes, Giggs, Becks, Keano, Bruce, Evra, Irwin, Cole, Ole, Wes, O'Shea, VdS, Rvp, Eric, Robson. Some are bigger legends some smaller but i rate them as legends.
From current players only De Gea.
And Martial