Roberto Soldado has said his struggles at Tottenham began when the London club sacked Andre Villas-Boas, the manager who signed him.
Soldado, 30, moved to White Hart Lane from Valencia in a deal worth £26 million two years ago.
But the Spanish forward scored only 16 times in 76 games and returned to Spain when he joined Villarreal in the summer.
Villas-Boas was sacked in December 2013, soon after Spurs had been heavily beaten at home by Liverpool, and the Daily Mirror quoted Soldado as saying that was his turning point.
He had scored nine goals in 17 games before the managerial change and said: "After the dismissal of Villas-Boas, who was the one who wanted me, everything went upside down.
"I lost confidence and had a hard time.
"But I think that experience made me appreciate and enjoy more what I'm living."
The striker's two goals in seven games have helped his new club to the top of the early Liga table, and he said: "It's vital to be happy in what you do.
"In the end, more than economic or sport reasons, it's key that you enjoy what you do.
"The most important thing for me was to enjoy things again, because after the two years I spent in England it was my goal to be happy again day to day."