Immobile joined the La Liga club on a season-long loan spell from Borussia Dortmund in July 2015, before signing on a permanent deal worth 11m euros.
The 26-year-old was then loaned to Serie A side Torino, where he scored five goals in 14 games last season.
Immobile made two appearances for his country at Euro 2016.

Nottinghamshire County Cricket Club is likely to receive the money from the city and county councils, along with Rushcliffe Borough Council.
Alan Rhodes, leader of the county council, said the money would be raised from a low-interest loan, which would be repaid by the club at a higher rate.
England beat Australia at Trent Bridge in August to regain the Ashes.
The city and borough councils have both already agreed to lend £2.7m each with County Hall due to make a decision on a similar figure at a meeting in October.
Two years ago the then Conservative-led County Hall came under fire for giving the club £900,000 towards a scoreboard.
At the time, Labour councillor Mr Rhodes described it as "insensitive" to be giving money while making cuts to services.
Since then, Mr Rhodes has become leader of the council but said this time it was different.
"We are providing that loan via our own lendings," he said.
"We are able to borrow money at a very low rate from the banks and we are able therefore to help Nottinghamshire County Cricket Club to be able to upgrade their facilities and they will pay that loan back with interest."
He added that the council could not take out similar loans to pay for ongoing costs such as care homes.
City council deputy leader Graham Chapman said the authority would get a higher rate of interest back on the loan than the current bank rate.
"We are going to get some money, they're going to save some money and we're going to get some very nice stands out of it," he said.
Nottinghamshire County Cricket Club would not comment before the final loan is agreed but it is understood the money would be used to revamp a stand and some facilities.
Work could start this winter.
Lazio have signed Italy striker Ciro Immobile from Sevilla for an undisclosed fee - reported to be around 8.5m euros (£7.1m).
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Trent Bridge cricket ground is set for a £8.1m revamp thanks to loans from three councils.