Pellegrini dismisses suggestions Perez refused to play
The victory was West Ham's first in the Premier League under the Chilean, but it also saw Marko Arnautovic suffer an injury and Perez attract attention on the bench.
Shortly after scoring West Ham's third goal, Arnautovic was forced off with a knee problem and Michail Antonio took his place.
Television cameras then picked up an angry-looking Perez engaging in what appeared to be a heated debate with a member of Pellegrini's coaching staff, leading to speculation the Spaniard refused to come on.
But Pellegrini explained that, while he planned to bring Perez on, he changed his mind once he saw Antonio standing ready to enter.
"It is very simple; when Marko had a problem with his knee, I wanted to put Lucas on," said Pellegrini.
"I thought he was ready to come on, but he was sitting on the bench. Antonio was ready so we were not delayed, so I put Antonio on.
"I didn't have to [explain the situation to Perez], I don't have to explain anyway, especially during the game, I can do that during the week.
"Antonio had to come in to help us win the game and he was the change."
Irons. pic.twitter.com/oS3ybn3UgR
— West Ham United (@WestHamUtd) September 16, 2018
Arnautovic's admission that he was in "a lot" of pain following his knee injury will have worried West Ham supporters, and Pellegrini was unable to reassure them.
"We'll see, I can't tell you know if it is [serious] or it's not," Pellegrini, who turned 65 on Sunday, added.
"It's behind his left knee, he felt something. Tomorrow [Monday] we have a recovery day and we will see what happens after that."