Wales are still waiting to learn the full extent of Gareth Bale's calf injury after the Real Madrid forward had a scan.
Real confirmed the 26-year-old suffered "muscle damage" but did not give a timescale for his absence.
Bale came off during Real's Champions League win over Shakhtar Donetsk on Tuesday.
Wales need one point against Bosnia-Herzegovina on 10 October or Andorra three days later to reach Euro 2016.
Bale was replaced by Mateo Kovacic after 32 minutes in the Bernabeu, with Real boss Rafael Benitez saying the extent of the injury would not be known until later this week.
"He had a spasm in the calf muscle. We have to wait one or two days. He had a similar problem last year," said the Spaniard.
Bale has scored six of Wales' nine goals during qualifying, with Chris Coleman's team unbeaten during the campaign and leading Group B by a point from Belgium and five from third-placed Israel - who they drew 0-0 with in the last match.
"I'd like to think that we've got enough to actually qualify without him," former Wales captain Kevin Ratcliffe told BBC Wales Sport.
"If it is a calf, my concerns were that in the last few games that I've seen Gareth play for Wales he has been stretching that calf.
"I'm not too sure if it's a long-term problem but hopefully it's not too serious and if it is an injury then maybe in two or three weeks he's going to be OK."