In a tweet from the Sumo Kyokai, it is confirmed that Ura has damage to his right knee Anterior Cruciate Ligament (ACL)
From a more medical web site
The knee is essentially a hinged joint that is held together by the medial collateral (MCL), lateral collateral (LCL), anterior cruciate (ACL) and posterior cruciate (PCL) ligaments. The ACL runs diagonally in the middle of the knee, preventing the tibia from sliding out in front of the femur, as well as providing rotational stability to the knee.
Full write up: http://orthoinfo.aaos.org/topic.cfm?topic=a00297
Suffice to say, Ura will be out of sumo for a while, and his road back will likely be long and hard, similar to what Tochinoshin faced in 2014, where he dropped to Makushita and had to fight his way back.