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 Coleman's Dragons slumped to the foot of the World Cup qualifying Group A table with a 3-0 defeat to Serbia in Cardiff.
Goals from Filip Djordjevic, Aleksandar Kolarov and Lazar Markovic put the game beyond Wales, who never looked like avenging the 6-1 mauling they suffered at the hands of the same opponents in Novi Sad a year ago.
Bale, 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 Real 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"
The defeat to Serbia increased the pressure on Coleman, who has agreed a two-year contract extension with the Football Association of Wales, but the former Fulham boss insists he is the right man to lead the Dragons despite them suffering six defeats from eight qualifiers.
He said: "I have every intention of leading this side into the next qualification campaign.
"We have two wins out of eight, which is not good but I am still excited about the group we could have available.
"The next campaign will be a better one."
He added: "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."