If you look at the history of the game, being a top player for the club hasn't been that much correlated with being a top manager in that club.
Sir Alex wasn't a United player, Sir Matt was a City ex-player and a Liverpool legend, Bill Shankly played for Preston, Mourinho didn't play at all, same about Sacchi, Herrera played for a bunch of clubs, neither of them being in Italy, Ancelotti was a Roma legend and while he played for Milano, he started managing them only after he gained experience in Reggiana, Parma and Juventus etc etc. Neither of these great football managers knew the club inside out or played for that club (bar Ancelotti).
However there are occurrances when a manager was a great player at that club. Cruyff and Guardiola are the best examples of that, but both were in unique scenarios. Cruyff as one of the most intelligent players of all time was way ahead of his time. If 'student of the game' means anything, then I doubt that there are many people who can claim to be it more than Guardiola. And finally, there is Bob Paisley with whom Giggs can have the most similarities. Like Giggs, Paisley was a legend of his club. Like Giggs, Bob Paisley was a part of the coaching staff at his team. Unlike Giggs, Paisley spent 20 years in those positions starting from a self taught physio, to a coach on the reserve team, to a coach in the main team, to assistant manager, to the man who was responsible for tactics during Shankly era and finally he became the manager. He didn't get the job cause he was a great player, he took the job cause he was a great manager.
Now if we can agree that being a top manager doesn't require knowing the club inside out, or even being a top player in first place (Pele, Maradona, Di Stefano, Best, Charlton, Platini, Eusebio, Garrincha are all top 10 players and whichever of them tried to do a job at management was shown to be an utter failure) then Giggs being a top player and a United legend becomes irrelevant in his likelihood of being a top manager.
Bow if we can talk about Giggs being a fantastic coach, a student of the game, having a wonderful knowledge of tactics (something that every top manager had), then there would be an interesting debate on that. But the closest we have seen Giggs talking for tactics is this:
And here you can see Giggs' charisma:
Now, compare it with Pep Guardiola: