Maxi Gomez and Iago Aspas struck for Celta's 2-0 win.
"This match was a good wake-up call, for me first of all," he said.
"We already had 10 not-so-good minutes at the end of the Rayo Vallecano game and the same thing happened to us at the start of the second half.
"It's completely my responsibility because surely I wasn't clear during the break.
"We didn't have a good game and Celta were clinical at the start of the second half.
"We did have situations in the first half where we could have chosen better, but we couldn't do it.
"Then in five minutes they settled the match and deserved to win."