Here's a sign Le'Veon Bell's holdout has probably gone on too long ...
Bill Peduto -- the mayor of Pittsburgh and a longtime Le'Veon supporter -- tells TMZ Sports he's beginning to think the Steelers don't need the RB to make a Super Bowl run.
"James Conner's proving to be a worthy back and it's a team sport," Bill says. "It's beyond one player."
It's a bit of a surprising claim ... considering Bell has nearly 8,000 yards from scrimmage in 62 career games ... but Bill -- like some of Bell's own teammates -- seems to be fed up with the holdout.