Borussia Dortmund star Erling Haaland must still convince Lucien Favre of his match fitness despite a sensational start to life in Germany.

Haaland arrived at Signal Iduna Park from RB Salzburg last month, billed as one of the hottest properties in European football, and the teenage striker has certainly not disappointed.

He scored a hat-trick from the bench to inspire a 5-3 comeback win over Augsburg on his debut and followed that with a brace in a 5-1 rout of Cologne.

The Norway international was at the double once more on his first start, with Union Berlin dismissed 5-0, although he was a substitute once more for the DFB-Pokal tie at Werder Bremen in midweek.

Naturally, Haaland scored after his introduction, although BVB fell to a 3-2 defeat.

Asked whether he regretted not starting the in-form forward, Favre kept his sights trained on the bigger picture at a Thursday news conference.

"As I said, I am the first one to [know whether to] let him play from the beginning," he said.

"You don't know his fitness like I do. So, we have to be careful. We have to be reasonable.

"Of course, I would like to let him play all games, but we still have to be careful with him sometimes.

"He is 19 years old and I speak about the pre-season and old injuries. We have to be reasonable."

Favre's hand might be forced, however, given his forward line will be without Marco Reus when they travel to face Bayer Leverkusen this weekend. 

"Unfortunately, Marco is injured now. It happened in the last minute. I am very sorry for him," said Favre, with his captain set for a four-week spell on the sidelines due to a muscular complaint that will cause him to miss the Champions League showdown with Paris Saint-Germain on February 18.

"But Marco is still very important and will be important for the team. There are several options how to replace him, but it depends on the system. Today it’s still not decided."

Emre Can, a deadline-day arrival on an initial loan from Juventus, made a substitute cameo against Werder and could make his league debut for Dortmund against old employers Leverkusen.

"We have to see him in training, how he feels. He did not play a lot [at Juventus this season]," Favre added.

"He made a good impression yesterday. The training was very long and intense. He did very well.

"He has to show in training what he is able to do. We know he can play as a defender with a back three but also in the midfield as six or eight, also in a 4-3-3.

"We will see because right now I cannot tell you exactly."