Germany coach Joachim Low feels Thomas Muller is going through a "difficult phase" as he backed the forward to return to his best.
But he remains part of Low's plans and the coach said Muller – the Golden Boot winner at the 2010 World Cup – could still play a key role for Germany.
"Although he hasn't stuck out lately at the national team or his club, Thomas Muller is a player who gives a lot of energy and can lead and animate the youngsters," he told a news conference.
"He still has those qualities of a great finish, of backing up. I have seen him in good form against [Borussia] Dortmund. Bayern's players were in a good form for 60-70 minutes.
"Thomas Muller is a player who will always be able to decide matches, although he is currently going through a difficult phase."
Low's men are facing relegation from League A and the 58-year-old accepted his team, stunningly eliminated in the group stage of the World Cup, were going through a tough period.
"To stay in the Nations League is out of our hands. We have to wait and see how Netherlands will play against France," he said.
"The most important thing for me are next year's [Euro 2020] qualifiers. Next year is important to find the right line-up, to get used to one another and to become more secure in the way we play.
"Of course we are in a period of upheaval. The match against France showed we are able to keep up. [The] result [against Russia] won't be of too much relevance.
"For me, it's about what we will get out of the match and what we will notice.
"We need to see how Netherlands will perform against France. We might have a chance to get through the group, depending on Netherlands' result.
"Then we will have to win this game, if possible."