Republic of Ireland coach Martin O'Neill says Robbie Keane's recovery from injury has been affected by his regular long-haul flights from the United States.
LA Galaxy striker Keane missed Ireland's 1-0 win against Switzerland on Friday and has also been ruled out Tuesday's game against Slovakia due to a knee problem.
Ireland's record goalscorer has not started any of his country's last five games and O'Neill said the 35-year-old's travelling is having an impact on his fitness levels.
"Robbie stayed out of the training this week because the little problem with the knee didn't seem to be getting any better," O'Neill said.
"I think that he feels that this is not a long-term thing but he was just slightly concerned because he thought he was over it.
"Of course, then he had the flight as well. It's a long flight, he's just stepped off the plane and maybe that didn't give it a chance to settle.
"I don't want to be scaremongering. Robbie comes in generally after a day's flight. He's as good a trainer and as enthusiastic as anybody. He is our captain, our best goalscorer and is good around the place."
O'Neill will make changes for Tuesday's match at the Aviva Stadium, with John O'Shea and James McClean among those coming into the side.
The Ireland coach is also considering a late call-up to add to his forward options, with Kevin Doyle, Jonathan Walters and Daryl Murphy all injured, leaving Shane Long as his only available striker.
Slovakia travel to Ireland on the back of a four-game unbeaten run, but Jan Kozak's side were held 0-0 at home by Latvia on Friday.
Vice-captain Marek Hamsik could be rested for Napoli's ongoing Serie A title battle with Juventus, with the midfielder left dissatisfied by Latvia's defensive approach.
"Latvia has come to defend and we did not manage to score," Hamsik said.
"The defence was very compact. Our goal was to win and to entertain the audience."