I think in the short term players like Cahill probably will be regarded as a "legend".
But because he never won a trophy as a player for us, younger fans won't learn about him, so eventually he will fade from memory. Which is sad really because he was a great player.
For me the true legends of the club have it all; ability, personality, trophies and an association with the club after they have stopped playing.
We all have our favourite players though, for different reasons, mine, when I was a kid was Southall, he was just effortless in goal, he is a true legend, I met him and wasn't dissapointed, nice fella.
I have also liked lots of players over the years, kanchelskis was one of the best players we had in the 90's, But wasn't here that long and didn't win anything with us. So as much as I like him, he is not a "Legend"
I think Ferguson is probably the last player to play for us that I would consider a "legend", won the FA cup with us, played over a long period (admittedly injured most of the time), his attitude on the pitch was amazing and involved with the club, coaching.