The Romanian was brought in to replace Joe Hart in City's goal and pulled off several fine saves to keep the score at 0-0 into extra time.
He could now replace under-fire Hart in City's Premier League game at the weekend.
Pellegrini was not surprised by his performance.
He said: "Pantilimon has always had my trust and that of the team.
"He is a good player, that's why he is here. He may not have played too much because Joe Hart plays last year and this year but I trust him to play.
"I think for him and the team to keep a clean sheet today was important."
Pellegrini insisted, however, that he has yet to determine Hart's fate.
Pantilimon was hardly overworked on Wednesday night but did everything that was required of him when called upon.
He got down well while partially unsighted to paw a Shola Ameobi effort wide in the first half and then, with the scores still level early in added time, the Romanian came out to meet Papiss Cisse and stood firm to make a strong save.
That latter intervention was picked out by home manager Alan Pardew as the key moment of the match, giving City the platform to secure passage to the quarter-finals via goals from Alvaro Negredo and Edin Dzeko.
The focus on the under-pressure Hart was inevitable, but Pellegrini did little to fan the flames around England's number one.
While he stopped short of guaranteeing a return to the starting XI against Norwich on Saturday, he also declined to anoint Pantilimon as his successor.
"He is a very good goalkeeper and he normally plays in the Capital One Cup and the FA Cup," said Pellegrini.
"This was a very important match for him. But I will start to think about the game against Norwich on Thursday. Today I was just thinking about winning this match.
"I think that we will talk tomorrow (about the team selection). One goalkeeper can play but the other will have our full support. We will think about it."
Pressed for further details about Hart's standing, Pellegrini responded curtly: "I have just told you I will think about that tomorrow."