Coach Gus Bradley said Gratz's injury, which occurred on the first play after the two-minute warning in the first half, was originally diagnosed as a shin contusion but X-rays and other tests revealed it to be more severe.
The 5-foot-11, 201-pound Gratz, whom the Jaguars drafted in the third round in April, was wearing a walking boot on Monday.
One of Gratz's strengths was his durability. He never missed a game at Connecticut and started 39 of the 50 games in which he played. He started against the Chiefs and had three tackles.