Dortmund s Erling Haaland reacts during a German Bundesliga soccer match between TSG 1899 Hoffenheim and Borussia Dortmund in Sinsheim, Germany, Saturday, Jan. 22, 2022. AP
The 21-year-old Norway star has scored 23 goals in 21 games for Dortmund this season.
However, after scoring the opening goal in Saturday's 3-2 win at Hoffenheim, Haaland crashed to the turf after a sprint and was left clutching his groin before being substituted.
On Monday, the exact nature of the injury and the length of his absence were still unknown.
"On Sunday and Monday Haaland underwent comprehensive examinations," the Bundesliga club said in a statement.
"According to the medical report, Haaland is suffering from muscular problems, which require treatment and further examinations in the coming days."
Haaland has already missed ten games in all competitions this season and Dortmund were knocked out of the Champions League last autumn when he was sidelined by a hip injury.
The Bundesliga has just started a two-week break and Dortmund want their star forward back as quickly as possible.
"We wish that Erling will be free of symptoms as soon as possible and do what he loves to do most: score goals for Borussia Dortmund," the statement added.
Speculation is rife that Haaland will leave Dortmund at the end of the season as there is reportedly a release clause in his contract, which runs until 2024, which became active at the start of 2022.
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