Borussia Dortmund newcomer Giannis Konstantelias will miss "several months" with a serious knee injury picked up in Saturday's home win over Hamburg, his club announced on Sunday.
Konstantelias impressed in his first German Bundesliga start by scoring Dortmund's second goal in the 2-0 victory, but was helped off the pitch early in the second half.
Dortmund coach Niko Kovac said after the match the club "feared the worst" and a follow-up examination on Sunday showed an injury to his cruciate ligament.
"Giannis Konstantelias has suffered a torn cruciate ligament in his left knee and will be sidelined for several months," the club said on social media.
The 23-year-old Greece international joined Dortmund from PAOK earlier in August for a reported fee of 30 million euros ($36 million).
The midfielder arrived despite protests from the Greek club's ultras, who occupied the stadium in the hopes of preventing the move, having stopped a similar sale to Bundesliga side Stuttgart a year earlier.