The Brazilian-born forward missed Wednesday's 1-1 Premier League draw at Liverpool and will not be back for Sunday's final game of the season at home to champions Leicester City.
"He needs a good rest and with training he might be fit for the Euros but I’m not sure," Hiddink told a news conference.
Spain coach Vicente del Bosque will name his initial 23-man squad on Tuesday, with the option to make changes by May 31.
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The European champions have been drawn in Group D with Croatia, the Czech Republic and Turkey at the Euro 2016 finals, which run from June 10-July 10 in France.
(Reporting by Steve Tongue; Editing by Ken Ferris)
