Arsenal manager Arsene Wenger revealed on Tuesday that winger Alex Oxlade-Chamberlain will be out of action "for a while" with a knee injury.
The 20-year-old English international hobbled off during Arsenal's shock 3-1 loss at home to Aston Villa on the English Premier League's opening day on Saturday, prompting reports he could be sidelined for up to six months.
Wenger did not go into detail about the likely duration of Oxlade-Chamberlain's absence, but it is an untimely piece of bad news ahead of Wednesday's UEFA Champions League play-off first leg away to Fenerbahce.
"The difference from Saturday is that Oxlade-Chamberlain is out for a while with a knee problem," Wenger told the club's official website before flying out to Istanbul with the squad.
However, there was better news on Bacary Sagna, Kieran Gibbs, Aaron Ramsey, Jack Wilshere and Tomas Rosicky, all of whom returned to training on Tuesday morning despite sustaining injuries during the defeat by Villa.
There were particular fears that Sagna had hurt himself badly after he landed awkwardly on his neck towards the end of the game, but Wenger said that all five players were in contention to face Fenerbahce, as well as left-back Nacho Monreal, who has been out with a back problem.
"Everybody else should make it," Wenger said. "There's a final decision (on Wednesday) but Sagna and Gibbs should be available. Nacho Monreal is back and will travel."
Arsenal's players were booed off after going down to Villa at the Emirates Stadium, but Wenger is confident they will respond in positive fashion by securing a berth in the lucrative group phase of the UEFA Champions League.
"We fought very hard last season to be in the top four and that game has a vital importance," Wenger said. "It is a consequence of what we tried to achieve last season and therefore, on the back of our defeat against Aston Villa, it's important as well to bounce back straight away.
"To be part of the top level - that is for us the most important thing. All the rest are the consequences of it. When you are a football player, you want to play at the top level and that's what the Champions League is about."