The 32-12 months-antique utility player, who can play in midfield or defence, arrived from league competitors Valencia in the wintry weather switch window after the departure of Englishman Kieran Trippier.
He was harm in stoppage time of Sunday's league match towards Barcelona, which Atletico lost four-2, after a hard address from former Valencia crew-mate Ferran Torres.
Wass finished the game for his overwhelmed group.
On Monday, the membership said checks revealed Wass had "a grade II sprain of the internal lateral ligament of his proper knee."
The assertion did not specify how long he would be out, saying it relied on the participant's recovery.
Spanish media predicted he will be sidelined for a month.
Atletico are two factors at the back of Barcelona in fifth in La Liga, and face Manchester United in the first leg of the Champions League round of sixteen.
"He is a completely versatile participant who can play in several areas of the sector," the membership stated on signing Wass.
Capped 41 instances for Denmark, Wass's career has protected stints with Danish membership Brondby, French aspect Evian, Benfica in Portugal and Spanish outfit Celta Vigo.
