Reports claim Bayern Munich are ready to spend €40million on the 23-year-old midfielder.
"I hope that he will do really well at Bayern, becomes one of the stars of the Bundesliga, and helps them to the league title. There's a big risk, though. I've been asking myself whether he's worth this kind of money, and I don't think that's the case," Effenberg told TZ.
"It has surprised me a bit that Bayern are so desperate to get him. He is not even a regular starter for Spain. Yes, he has won the World Cup and the Euros, but he played only 25 minutes at Euro 2012.
"It's clear that he's not on par with the big guns like (Andres) Iniesta and Xavi. That's why I'm wondering whether he's worth the money."