Garnacho had entered the game in the 16th minute as an emergency replacement for the injured Estevão Willian, but his performance was badly exposed.
He failed to track Bruno Fernandes' run down the right flank that led directly to Matheus Cunha's winning goal just before the break, and was repeatedly dispossessed by Diogo Dalot throughout the first half.
United players Noussair Mazraoui, Casemiro and Bryan Mbeumo applauded Dalot after one particularly notable dispossession. Garnacho lost possession 15 times across 74 minutes and failed to register a single shot, assist or accurate cross.
Cole Palmer was also among those said to have had words with Garnacho at half-time., reports The Sun. Chelsea insist no player was singled out during the break.
The match carried additional edge given how Garnacho left United last summer, following a public falling out with former head coach Ruben Amorim after the Europa League final. He had criticised Amorim in a post-match interview and posted a photo wearing a Marcus Rashford Aston Villa shirt, both of which sat badly in the dressing room.
United players had lobbied for Garnacho and other disruptive figures to be excluded from the pre-season tour, and none of his former teammates greeted him at full-time on Saturday.
Luke Shaw posted a photo challenging Garnacho on Instagram, which was liked or commented on by Dalot, Fernandes, Cunha, Joshua Zirkzee, Ayden Heaven, Lisbandro Martínez, Patrick Dorgu and Mason Mount. The club also posted clips of their tackles on the Argentine.
Garnacho has scored eight goals in 40 Chelsea appearances but has not started in their last six games. Chelsea are already exploring options to sell him this summer for around £40 million — the same fee they paid for him.