Wales manager Chris Coleman believes Real Madrid would be taking a major gamble if they were to include Gareth Bale in their starting line-up on Saturday.
Real's trip to Villarreal is expected to mark Bale's first appearance in the club's famous white shirt since completing his world-record move from Tottenham at the end of the transfer window.
But the 24-year-old did not feature in Wales' 2-1 defeat in Macedonia on Friday after picking up a groin strain, and underwent a scan on Monday.
He was later able to take a full part in Wales' training session ahead of Tuesday's World Cup qualifier against Serbia, but a final decision over his fitness will only be made on the night.
If Bale plays no part in Cardiff, his previous match action will have been back in mid-July.
And Coleman does not believe Bale can start at El Madrigal on Saturday given his lack of training and game time.
Coleman said: "To have four sessions and then play a game like we have got is tough. La Liga is a different tempo to the Premier League, but I'd be surprised if he starts.
"I understand they've got a problem with Cristiano Ronaldo, but it's a big, big ask to do four training sessions [and then play].
"He is a super athlete, he's not got an ounce of fat on him, but even for him, if you see him in the sessions with us - and we're not full pelt - he's not quite there.
"He's only had four sessions and one of them was by himself with our physio last month, before the Republic of Ireland game. But it will be up to him. If he says he can play a part, he'll be on the pitch no doubt."