After being injured in training, Bellerin missed Betis' key match against Atletico.
In the squad list released by Betis on Saturday, there was no Bellerin name.
Pellegrini said in a pre-match press conference that Bellerin and several other injured players could not play in this game.
Bellerin's injury was supposed to have occurred in training, as he did not play as a substitute in last Thursday's match against Vallecano, who had previously played his full game against Sevilla. There were no signs of injuries before the weekend.
Bellerin has been very busy this season, making 26 appearances in all competitions.
It was also a successful season for him and the team, who were at one point in the top four of La Liga, reaching the Copa del Rey final and advancing to the Round of 16 of the Europa League.
Unfortunately, Betis lost to Atletico Madrid in the fourth round and lost this crucial game.
Chambers surprised many with a move to Aston Villa in the winter window, where the defender contributed a brilliant pass in a 4-0 win over Southampton that was dubbed "Chamberlinho" by fans.
While playing for Arsenal, Chambers had five goals and 10 assists, but few Arsenal fans could notice a better pass than the one he sent for Villa on Saturday.
He lifted the ball with the instep of his outer foot and went through the Saints' backline, where he accurately found Coutinho, who had a simple task, passing the ball to Douglas Luiz and extending the score to 2-0.
In the five Premier League games since joining Villa, Chambers has played three of them.
It was the first game Villa won when he started, after losing 1-0 at home to Newcastle and Watford respectively.