Leeds centre-backs Liam Cooper and Diego Llorente have both picked up injuries on international duty, casting doubt over their fitness for the Premier League's return.
Spain confirmed on Sunday that Llorente had withdrawn from the squad and will not travel to Kiev for the clash with Ukraine on Tuesday after picking up a muscle problem.
He was an unused substitute for the win over Switzerland on Saturday and Leeds have been informed of his knock.
The Whites skipper Cooper, meanwhile, was a surprise omission from Scotland's team to face Slovakia on Sunday and boss Steve Clarke confirmed he had picked up an injury and will miss the game against Czech Republic later this week.
"Unfortunately Liam Cooper has picked up an injury and he will be out the next two games," he said.
Cooper missed the opening Premier League game of the season against Liverpool but has started the three matches since, including the draw with Man City last time out. Leeds face Wolves next a week on Monday.
He played 120 minutes last Thursday night against Israel as Scotland went through to their final Euro 2021 play-off game against Serbia via a tense penalty shootout win.
Llorente, who is yet to play for Leeds, bagged his sixth cap for Spain when he started in the friendly against Portugal last week.
He was an £18m signing from Real Sociedad in the summer, following the promotion of Marcelo Bielsa's side from the Championship to the Premier League.
Cooper, meanwhile, has been at Elland Road for over six years since signing from Chesterfield.