Maxi Gomez opened the scoring for Celta after they had a goal ruled out by the video assistant referee (VAR).
VAR later awarded Celta a penalty and Iago Aspas converted from the spot.
"We dominated the match at moments," he said.
"Their first goal came and it gave them the option to go on and win the game.
"We had no answer.
"We played with a lot of new faces and in the second half we were unable to generate what we needed to score."