Interesting to see what the official club website has to say on this matter. The definition and criteria seems to be matches played for the club. ManUtd.Com lists the following players as legends:
Ryan Giggs Appearances: 778 Goals Scored: 146
Sir Bobby Charlton Appearances: 758 Goals Scored: 249
Bill Foulkes Appearances: 688 Goals Scored: 9
Paul Scholes Appearances: 582 Goals Scored: 139
Gary Neville Appearances: 554 Goals Scored: 7
Alex Stepney Appearances: 539 Goals Scored: 2
Tony Dunne Appearances: 536 Goals Scored: 2
Denis Irwin Appearances: 529 Goals Scored: 33
Joe Spence Appearances: 510 Goals Scored: 168
Arthur Albiston Appearances: 485 Goals Scored: 7
Roy Keane Appearances: 480 Goals Scored: 51
Brian McClair Appearances: 471 Goals Scored: 127
George Best Appearances: 470 Goals Scored: 179
Mark Hughes Appearances: 466 Goals Scored: 164
Bryan Robson Appearances: 461 Goals Scored: 98
Martin Buchan Appearances: 456 Goals Scored: 4
Jack Silcock Appearances: 449 Goals Scored: 2
Gary Pallister Appearances: 437 Goals Scored: 15
Jack Rowley Appearances: 424 Goals Scored: 212
Sammy McIlroy Appearances: 419 Goals Scored: 72
Steve Bruce Appearances: 414 Goals Scored: 51
Source:
Code:
http://www.manutd.com/default.sps?pagegid={12B35B6C-4309-4553-AEF4-050A3B899C97}§ion=appearances&teamid=1445