It's been suggested Pellegrini could yet leave Betis at the end of the season, despite European qualification now all but guaranteed.
Isco said: "He's been there for two of the most important moments for a player, at least at the beginning.
"He gave me the opportunity at Málaga and I played every game. He's always given me a lot of confidence to develop my personality and my style of play on the pitch. And now at Betis, it's the same.
"Since I arrived, I'm enjoying myself like the kid who started out in Málaga. As for his staying on? I just can't imagine Betis without Pellegrini. What he's done at the club is unprecedented.
"Since he's been here, along with Barça, Atlético and Real Madrid, Betis is the team that has qualified for Europe for the most consecutive seasons. And that's very important for a club like this with fans like this."