Ripped By Giants Prospect After HR

Fernando Tatis Jr.'s first homer of 2023 didn't go as well as he would have liked ... he was booed as he rounded the bases -- and then the pitcher who served it up straight up called him a "cheater."

The San Diego Padres star hit the dinger on Wednesday night while down in the minor leagues as he continues to serve his 80-game ban for his positive PED test last season.

The 24-year-old, playing for the El Paso Chihauhaus against the Sacramento River Cats, smacked the baseball deep over the left field wall ... and seemed rather impressed by the tater.

He watched it fly -- then skipped around the bases ... but there weren't many outside of him at the Northern California ballpark who were thrilled…