Remember ... the Texans star fractured his tibial plateau back in an Oct. 8 game against the Chiefs -- and he's been working like a mad man to get back to the field.

While showing off his new "Reebok JJ II's," Watt also put the scar -- which looks like it's about half a foot long!! -- on display ... and described his grueling comeback process.

"You have to work and grind to rebuild," Watt says.

"It's dirty. It's hard. It's not easy. It's not fun. But, there's beauty in the struggle. It's time for a fresh start."

Judging by J.J.'s most recent trip to the gym ... we're pretty sure that "fresh start" is gonna come with a whole lot of ass-kicking.