The 26-year-old was once regarded as one of the best talents in La Liga during his time with Barcelona, but his career stalled before moving to Sporting in 2011.
The forward struggled to make his mark in Portugal, however, and has now returned to Spain.
"I've come here to help Pucela and succeed. My time in Portugal was very bad but I live in the present and for me, it's a wonderful to be here. In a footballing sense, I feel that being here, I can back to my former form again," he told reporters at a press conference.
"There is a lot of quality on the squad, the team can play with one or two touches. I feel like we can continue in the Primera."
A number of Spanish clubs were believed to be in the race for his signature.