Garnacho, who left United for Chelsea on-loan, was kept on the bench by manager Enzo Maresca for the 2-1 defeat.
He was jeered throughout the game by fans, who accused him of lack loyalty. But senior United players felt the Old Trafford crowd were too harsh on their forme teammate.
A source told The Sun: “Garna is a very talented boy but there is no denying he could be a pain in the a*** while he was at United.
“A few of the more experienced players tried to talk to him and sometimes he listened, sometimes he didn’t.
“That said, a lot of the players felt sorry for him for the reception he got at Old Trafford. It wasn’t a surprise, but they just think he’s a young kid who’s been badly advised by the people around him.
“Plenty of players have left United in worse situations but he got an awful lot of stick.”