The Spanish international failed to make an impact at the London club after signing from Liverpool for £50m.
The 27-year-old, who scored once in 18 appearances for the Blues last season, previously stated that he will be prolific for the former Premier League champions this term.
Despite the striker’s claims, Birtles is not convinced that Chelsea fans will get the best out of the former Liverpool star.
"I don’t think he wants to be where he is at the moment," Birtles told talkSPORT. "I get the impression he wants to be back in Spain.
"I said that last season, you watch his body language and he doesn’t look like he wants to be in this country. He looks like he wants to be back playing where he was successful before.
"We know what he can do. Look what when he first came to Liverpool, he was absolutely stunning. He glided across the surface, he had every finish in the box.
"He's clearly a really talented player but I just hope his mind and body is in London and he wants to play for Chelsea because, if he doesn't, you're not going to get the best out of him."
The Blues begin their Premier League campaign away from home against Stoke on August 14.