Betis were beaten 1-0 via Steve Kapuadi's winner for the Poles on Thursday.
Afterwards, Pellegrini admitted: "Performances are not always the same. We have a squad and we have to play in all three competitions with that one. We have little creativity without Isco, William, Lo Celso and that was what was missing today. We had the desire but we were not able to hurt them.
"We lacked creativity, we did not generate shots or much danger. You cannot draw any clear conclusions from one game. Hopefully we will qualify, which is the objective.
"We lacked creativity, the team tried to play but couldn't find space in a tight defence. We didn't have that today, we tried with forwards on both wings but we didn't have any options."
On losing ahead of the derby against Sevilla, Pellegrini continued: "I don't think this game will affect the derby, whether we win or lose. On Sunday we will go looking for the game from the start, today we tried and we were not creative.
"We were very good in defence, especially the centre-backs. The game ends today, we will try to qualify in the next games and I expect a Betis like the one we have seen in the previous games. It was quite strange that we did not create any chances today."
On Legia, he added: "We know that they are strong at home and have a fan base that supports them for 90 minutes. We knew about the quality of their players and we had a certain level of security, at least defensively, which helped us to try to get the equaliser. These are three very important points for Legia at home that will help them to qualify."