Wales manager Chris Coleman believes it would be "dangerous" if Real Madrid attempt to play Gareth Bale for 90 minutes against Villarreal on Saturday.
Bale played 32 minutes as Wales lost 3-0 to Serbia in Cardiff to slump to the foot of the World Cup qualifying Group A table.
The Real Madrid star, a pre-match doubt due to a groin problem, appeared just before the hour to get his first action since becoming the world's most expensive player when he joined the Spanish giants from Tottenham earlier this month, and almost grabbed a goal with a superb swerving, dipping free-kick.
The 24-year-old will now return to Spain ahead of his expected club debut, but Coleman believes his star man is still some way short of fitness given he has played just 32 minutes since mid-July.
He said: "If he starts Saturday, he will not finish the game. I would be amazed if he does.
"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."
He 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."
Bale himself tweeted after the game: "Great to finally get back to playing tonight, shame about the result though. Can't wait to get to Madrid tomorrow £halamadrid"