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The 24-year-old agreed a four-year deal just 24 hours after withdrawing from an agreement with the Scottish champions.
A switch to Getafe collapsed, allowing Celtic to move again for the forward who scored 22 goals in the Spanish second tier last season.
And Manchester City striker John Guidetti could also join up with the Parkhead side on loan.
The paperwork for the Swede's switch to Celtic was not filed with Fifa until after midnight on deadline day, but the SFA are in the process of appealing to the game's governing body to register the deal.
Guidetti, in the final year of his City contract, was a prolific goalscorer during the 2011/12 campaign on loan at Dutch side Feyenoord.
A January move to Stoke last season was not so productive, with the striker making just six appearances as a substitute and failing to find the net.
Meanwhile, Celtic have allowed forwards Teemu Pukki, Amido Balde and Holmbert Fridjonsson to move away on loan as manager Ronny Deila reshapes the squad he inherited from Neil Lennon over the summer.
Pukki and Fridjonsson have joined Danish club Brondby, while Balde has gone to Belgian club Waasland Beveren.
Fringe midfielders Dylan McGeough and Jackson Irvine have moved on to Hibernian and Ross County, respectively.
Pukki, 24, signed a four-year deal in August 2013 but the Finland international has not been a regular starter, managing seven goals during his time in Scotland.
Balde, 23, is also under contract until 2017 and the former Vitoria Guimaraes striker has not featured at all this season after making just three starts in the last campaign.
Jo Inge Berget, Aleksander Tonev, Jason Denayer and Wakaso Mubarak have all arrived at the Scottish champions on loan this summer, while free agent Craig Gordon was Deila's first signing.
Celtic have completed the £2.3m signing of Serbia striker Stefan Scepovic from Sporting Gijon.