Erling Haaland is in contention to start in Borussia Dortmund's clash with Cologne on Friday but head coach Lucien Favre said the club's new striker must "keep working on his basic endurance".
The €20million signing from Salzburg scored three goals on his debut as Dortmund came from behind to beat Augsburg 5-3 on Saturday and Favre said there is a chance Haaland could break into a team that started with Jadon Sancho, Marco Reus and Thorgan Hazard in attack last time out.
Prior to his 34-minute appearance against Augsburg, Haaland had not played a competitive game since Salzburg's 2-0 defeat to Liverpool on December 10 and Favre reminded those calling for him to play from the start against Cologne that he is short of fitness.
"Erling Haaland made a great impression," Favre told reporters. "He always trains at full throttle, he always wants to win.
"If he misses a chance to score, he gets angry. It's great to train players like this - it's good for everyone.
"But he hasn't trained for a long time. He has to keep working on his basic endurance. We should not forget that.
"He is focused on his tasks, he is always in a good mood. He offers a reference point for the midfield. But he is only 19 years old."
Gio Reyna also made his Bundesliga debut against Augsburg, coming on as a substitute for Hazard in the 72nd minute, and Favre tipped the 17-year-old midfielder to push for a regular first-team place in future.
Reyna joined Dortmund from New York City and has made 11 appearances for the Yellow and Black Under-19s this season, scoring four goals.
"In training you can see that Gio Reyna has something special," said Favre.
"If you can't see that, you're blind. At 17, he does everything very well.
"If he continues to play at this level, he will make great strides."