Wales manager Chris Coleman has said it would be "dangerous" if Real Madrid attempt to play Gareth Bale for the whole of their match against Villarreal on Saturday.
Bale was on the field for the final 32 minutes of Wales' 3-0 defeat by Serbia in Cardiff on Tuesday, a result that dropped Coleman's men to the bottom of the European zone World Cup qualifying Group A table.
The 24-year-old had been a pre-match doubt because of a groin problem.
But Bale, who joined Real Madrid from Tottenham Hotspur last week in a deal widely reported to be the world's most expensive transfer at around £86 million (100 million euros, $132.5 million), looked sharp in his first competitive action since the move and nearly scored a consolation goal with a swerving free-kick.
Bale now heads back to Spain, with Coleman warning he was still short of full fitness given Tuesday's appearance was his first match action since July.
"If he starts Saturday, he will not finish the game. I would be amazed if he does," Coleman said of Bale.
"We have looked at him in these 10 days and he is nowhere near ready for 90 minutes even if Spanish football is slower.
"He may start, but he won't finish the game.
"He may get 45 minutes, I don't know, but he will feel better in himself after 30 minutes here.
"But he has only had four training sessions in two months which is dangerous."
Coleman added: "He played 30 minutes but after 15 minutes he looked tired -- but he has only had four training sessions, so you will not get what you normally get.
"But he gave us a lift, the fans were excited and it gave us a spur, although the game was done and dusted.
"We needed to be in the game when he came on. It was lost by then, but it was nice for the Welsh supporters to see him."
After the match, Bale told his Twitter followers: "Great to finally get back to playing tonight, shame about the result though. Can't wait to get to Madrid tomorrow £halamadrid"."
Meanwhile Tuesday's loss increased the pressure on Coleman, who has agreed but not yet signed a two-year contract extension with the Football Association of Wales even though the former Fulham manager has presided over six defeats in eight qualifying matches.
However, a defiant Coleman insisted he was the right man to lead Wales into the Euro 2016 qualifying campaign, where they will try to reach their first major finals since the 1958 World Cup with their hopes of playing in Brazil next year long gone.
"I have every intention of leading this side into the next qualification campaign," Coleman said.
"We have two wins out of eight, which is not good but I am still excited about the group we could have available.
The contract has been agreed, although I have not signed it, but if I thought I could not affect this group of players I would not waste my country's time.
"We can do something special, with a fresh run at it."