Barcelona said that Lionel Messi missed training on Monday with gastroenteritis, following the 1-0 defeat to Real Socieded this weekend.
Messi was a half-time substitute for Munir El Haddadi at the Anoeta stadium, but could not bring the Catalans back into the game after Jordi Alba's second-minute own goal had given David Moyes' side the lead.
However, a statement on the Barcelona website has confirmed the reason for Messi's absence and could see him doubtful for the Copa del Rey clash with Elche on Thursday.
It read: "Rafinha and Leo Messi missed Monday morning's open doors training session. The Argentinean has an upset stomach and Rafinha is suffering from an ear infection. The Club doctors will decide over the coming days when they are able to resume training."