The Aston Villa left-back went down in the 72nd minute as he went to make a challenge in France Under-21s' 1-0 win over Northern Ireland on Thursday
Amavi went to hospital following the fall and Mankowski says they must wait to discover the extent of the damage.
"Our doctor and that of Guingamp are travelling with him," he told a press conference after the game
"The first information that we have available is that it is a knee sprain
We don't know yet if the ligaments are ruptured
It's too soon to say that."
Amavi is still in France but, should it be serious, the injury would be a major blow to new boss Remi Garde
The 21-year-old joined Villa from Nice in the summer as part of Tim Sherwood's summer rebuilding but Garde replaced Sherwood earlier this month with Villa bottom of the Barclays Premier League
Amavi has made 12 appearances and was recalled by Garde for his first game in charge as Villa drew 0-0 with Manchester City to end a run of seven straight defeats on Sunday
Source : PA