Tests also confirmed that he tore his LCL, making the injury more significant because Mathieu has to wait for the injury to heal a bit before he can undergo surgery, Rapoport reported Monday.
Arians later confirmed the ACL and LCL injuries and added that Mathieu's rehab "will be a long one." The coach didn't expect to have Mathieu back by training camp.
Mathieu was injured in the third quarter after getting his left leg tangled while returning a kickoff.
The news is devastating for Mathieu, who was having a fantastic season and had vaulted himself into the lead for the Defensive Rookie of the Year award.
Arians said he still believes Mathieu has earned consideration for the award, saying, "It'll take two guys to take his place."