"He is a legit MVP candidate," Hairston told TMZ Sports before Tuesday night's game at Dodger Stadium ... a 12-0 L.A. blowout that saw Bellinger slug home run #22 (his 10th in 10 games!).
Hairston knows what he's talking about -- he played in the bigs from '98 to '13 and won a World Series with the Yankees in 2009.
As for Bellinger ... he's on the most torrid HR pace for a rookie EVER ... pretty insane considering the kid started the season in the minors.
Hairston -- a third-gen MLBer -- told us he's "never seen a start to a career like this."
Even Cody's dad, a three-time World Series champ, can't believe what he's seeing.