Former MLBer Kyle Farnsworth is getting ripped for his take on Orioles outfielder Trey Mancini ... after the ex-pitcher called out the star's toughness in his first season back after battling cancer.

29-year-old Mancini -- who beat stage 3 colon cancer in September 2020 -- has played in 128 games during his miraculous comeback in 2021 ... but took Monday off after getting hit in the foot and experiencing some "general soreness."

While most people wouldn't care what the 43-93 Orioles are doing in September, Farnsworth was randomly pissed about Mancini sitting out ... tweeting out some unnecessary criticism (without actually name-dropping him).

"So I just saw that a MLB player didn’t play today because of general soreness," Farnsworth tweeted.

"Are you kidding me. It’s September. Everyone is sore. If you can’t play through soreness, you shouldn’t be playing the sport. SMH!"

Of course, everyone knew who Farnsworth was talking about ... and the replies started to flow in.

"My man @TreyMancini is a living legend," Mets pitcher Marcus Stroman said Tuesday. "A true role model to the youth who has battled an incredible amount of adversity."

"Past players with poo-poo careers/opinions need to keep it down. He’s a cancer survivor who beat the odds and is a blessing to society. Keep inspiring Trey!"

FOX analyst Ben Verlander added ... "This tweet was in regards to Trey Mancini. I just can’t fathom sending this. You’re referencing the guy that beat Stage 3 cancer and sat out all last year due to cancer treatment? That’s really who you’re talking about sitting out a game? Wow.."

Mancini has 21 homers, 66 RBI and a .262 batting average this season ... and has been one of the few highlights for the Orioles this season.

Farnsworth has yet to delete the tweet -- there's still time, Kyle -- and has not addressed the backlash ... and it's unclear if he'll do either.