Germany coach Joachim Low admits he does not know when captain Bastian Schweinsteiger will be fit enough to return to training.
The Manchester United midfielder was included in the provisional squad for Euro 2016, despite being restricted to just six appearances for the club in 2016 due to knee ligament injuries.
Schweinsteiger said this week that he felt "right on schedule" in his recovery and fully expects to prove his fitness before Low trims his squad for France to a final 23.
But while centre-back Mats Hummels is progressing well following a calf problem, Low concedes that Schweinsteiger's return is more difficult to predict.
"With Hummels, it's clear when he comes back. Certainly he will need a few more days," he said ahead of the friendly with Slovakia in Augsburg on Sunday. "With Bastian Schweinsteiger, we don't know when he will be back in training with the team."
Low confirmed that a number of first-team stars will be rested for Sunday's game, with some – including Manuel Neuer, Thomas Muller and Mesut Ozil – left out of the travelling party altogether.
Youngsters Julian Brandt, Julian Weigl, Joshua Kimmich and Leroy Sane could all be given a chance to impress, but Low is eager not to affect their performances by putting them under pressure to force their way into the final Euro squad.
"Bernd Leno is our goalkeeper tomorrow for the first 45 minutes. For the second half, Marc-Andre ter Stegen will be in goal," he said. "Hummels, Karim Bellarabi, Neuer, Muller, Ozil and Marco Reus will stay here. All the other players will travel to Augsburg.
"I plan to use everyone I haven't used yet, so Julian Brandt, Julian Weigl and Joshua Kimmich – at least one will get used. Leroy Sane will play at least 45 minutes.
"I won't put too much pressure on them. They shouldn't lose the looseness they have amid all the tension. You can make mistakes that way. Even if it's not enough for one or two players to go to the Euros, we'll certainly see them with us soon afterwards."
Low expects a difficult test against a Slovakia side that he believes will serve as a useful reference point for group games against Poland and Ukraine next month.
"They are similar to our group opponents, Poland and Ukraine. They're defensively very well organised and fast in behind. They also have excellent counter-attacking players," he added.
"They're a good team and they have rightly qualified for the Euros. They will challenge us."