Betis won 2-0 via goals from Ez Abde and Vitor Roque.
Pellegrini said of Roque afterwards: "He's powerful, he gets into spaces a lot, he's fast. The goal is the typical goal of a goalscorer, from the rebound that he's waiting for.
"The great goalscorers have always come from rebounds. He believed until the end that he could keep pushing. I'm happy for him. He came from two games in Brazil and 30 minutes were enough for him and (Giovani) Lo Celso."
Assane Diao was impressive on the night and Pellegrini also said: "I see him as more of a centre forward. He had been training very well with the team. Today it was a bit more difficult for him but he has a very clear idea, that's why he's still in the squad.
"He wants to keep improving and we'll soon see him at a very high level."