Real Madrid said farewell to coach Juan Ramon Lopez Caro on Thursday after Racing Santander announced he was joining them next season.
"Obviously Racing are a humbler club with one of the smallest budgets in the league, but the demands will be the same," the coach told a news conference.
"After five years here I am sad to leave. I ask for forgiveness from the Real fans who had confidence in me, because professionally I was not able to rise to the occasion."
Lopez Caro was promoted from Real's B team to replace the sacked Vanderlei Luxemburgo last December and led Real to second place in the Primera Liga to earn them a place in the Champions League group stage.
Real have not yet appointed a replacement for next season and are distracted from the task of looking for a new one with a presidential election planned for July 2.
Racing were steered to safety by caretaker coach Nando Yosu as they finished in 17th place, one point above the relegation places.