Justin Fields acknowledges his knee isn't fully at where he wants it to be right now and puts himself at 80-85 percent. (0:32)

SCOTTSDALE, Ariz. -- Ohio State quarterback Justin Fields might not be fully healthy when the second-ranked Buckeyes face No. 3 Clemson in the PlayStation Fiesta Bowl.

Fields said Tuesday that his left knee is only 80 to 85% heading into Saturday's College Football Playoff semifinal in Glendale. He plans to wear a small brace during the game and brought a larger one in case he needs it.

"My knee's probably not where I want it to be right now," Fields said. "But I think with treatment every day and just resting it every day, hopefully, it will be better by the game."

Fields aggravated a previous injury during a game against Penn State on Nov. 23 and again the following week against Michigan when a teammate rolled into his knee. He struggled in the first half of the Big Ten championship game against Wisconsin before throwing two touchdown passes as the Buckeyes rallied for a 34-21 victory.

Fields has been practicing during the buildup to the Fiesta Bowl, but with limited mobility.

"I try to get as many reps as possible," he said. "I really haven't missed any reps, I've just been getting in there and playing. I just can't really move like I want to."

Fields finished third in the Heisman Trophy voting and was named the Big Ten Offensive Player of the Year after throwing for 2,953 yards, 40 touchdowns and one interception.

The winner of Saturday's game will face No. 1 LSU or No. 4 Oklahoma in the national title game Jan. 13 in New Orleans.