Gareth Bale, the world's most expensive player, has gone for a scan after suffering a groin strain in training before Wales lost 2-1 to Macedonia on Friday -- meaning he may have to wait for his Real Madrid debut.
Bale, an unused substitute in the Macedonia defeat, did not play any part because of that injury, Wales manager Chris Coleman said.
Coleman said the scan would determine whether Bale would have any involvement against Serbia in Cardiff on Tuesday.
"We had a conversation with Gareth before the game -- he had a tight groin from the day before and we could not take that chance," he explained.
"As we speak, he has gone to have another scan on it to clear his mind so he knows everything is alright.''
Real have been hoping that Bale -- who moved to Spain for a reported fee of £86 million -- can play in their next league game, against Villarreal on September 14. Cristiano Ronaldo is a fitness doubt for that match.
Some reports had indicated that Madrid would ask Wales to leave him out of their qualifiers against Macedonia and Serbia.
But Bale had stressed he wanted to play, saying: "I haven't had a lot of match time so that will help my fitness, as well as it being an honour to play for my country.
"It's well-known I had a slight injury, which took me a bit of time, but I always wanted to be here in the white shirt of Madrid and hopefully I can get match time with Wales and come back fit. I'm hoping to be involved for the game with Villarreal."
But the Welshman said he realised his world record fee was no guarantee of a starting place in the Madrid line-up, adding: "I can't come in and just walk into the starting 11. I've always wanted to improve, and there's no reason that should stop now I'm here. I can't wait to get started."