Tony Bennett was a great college player (he won the award for best college player in the country 6' or under. He played 3 years in the NBA. Larry Krystowiak (Utah), and Fred Hoiberg (Iowa St. - Bulls) were both great college players and solid NBA players. Coaching vagabond, Larry Brown was a big time college player at UNC, played on the US Olympic team, and was an ABA all-star. UCONN coach Kevin Ollie was a very good college player, and played a number of years in the NBA before winning a NC at UCONN as a coach. Steve Alford was a great college player, played for the US Olympic team and played in he NBA. Billy Donovan was a great college player at Providence, and played one year in the NBA.
In the NBA, there is a long list of good/great players that also were successful coaches: Steve Kerr, Larry Bird, KC Jones, Pat Riley, Bill Russell, Billy Cunningham.
Great basketball players can be great coaches, but obviously, great coaches don't need to be great players. People are grasping if they are legitimately claiming that Manning won't be a successful coach because he was a great player.