Johnny Manziel thinks Darren Rovell is a "bitch."

Yep, things are heating up between the ex-NFL star and the business reporter -- and it's all playing out on social media.

Here's how it started ... Rovell posted a tweet on Wednesday comparing 2 Sports Illustrated cover stories from 7 years ago.  ... featuring the Alex Rodriguez steroid scandal as well as the "Johnny Football" craze.

Rovell says A-Rod was at "Rock Bottom" back then while Manziel was the "envy of the college football world."

He then points out that in 2020, the tables have apparently turned.

"Today, A-Rod is as relevant as ever. Manziel achieves a flash of relevance by flipping off a building into a pool."

Side note -- it was a pretty impressive flip.

Manziel clearly feels Rovell's tweet was an unwarranted cheap shot -- and fired back Wednesday with one of his own.

"Seven years later and you’re still as big of a bitch as ever."

OUCH.

Rovell hasn't responded to Manziel and when we reached out, Rovell issued a "no comment."

Shocker, there's some bad blood between the two men that goes back years -- and it apparently all started in 2013 when Rovell investigated whether Manziel was signing autographs for cash while at Texas A&M.

Turns out, Rovell was on to something -- and Manziel was suspended by the NCAA for one-half of a football game.

He eventually went on to be a 1st round pick in the NFL Draft -- but everyone knows what happened after that ... rehab, CFL, fired from CFL, divorce, mental health issues, etc.

But 27-year-old Manziel has said he's moved on with his life and accepts that his NFL career has ended.

The argument that's broken out on social media ... is Rovell out of line for pointing it Johnny's downfall, knowing what he's been through? Or is it fair game since he's pointing out facts.

As for A-Rod, he's doing great -- he survived the steroid scandal. He made a fortune in business. He's trying to buy the NY Mets. And, he's engaged to J Lo.