2018 got off to a rough start for Northwestern football.

The school announced on Tuesday that, as feared, quarterback Clayton Thorson suffered a torn ACL in his right knee during the Music City Bowl last Friday. No timeline was given for Thorson’s return, but the redshirt junior will have surgery in the next two to three weeks.

Thorson injured in his knee at the end of a successful 25-yard trick play early in the second quarter of the Wildcats’ 24-23 win over Kentucky. Thorson caught a pass from Jeremy Larkin and rumbled down to the 5-yard line, when his knee buckled on a tackle by Kentucky linebacker Jordan Jones. Thorson was carted off the field and did not return to action.

Head coach Pat Fitzgerald said immediately after the game that an initial exam of the knee didn’t indicate a catastrophic injury, and he speculated that it may have been a hyperextension. But an MRI back in Evanston revealed that the injury was the worst possible scenario.

Players typically take anywhere from nine to 12 months to return from a torn ACL. That would peg Thorson’s return in September at the earliest and possibly much later. The Wildcats open the 2018 season against Purdue on Aug. 30, making it possible if not likely that a quarterback other than Thorson will start a game for NU for the first time since the end of the 2014 season.

Thorson has started every game over the last three years for the Wildcats and already claims school records for wins (27) and touchdowns (44). He just announced on Dec. 19 that he would return for his fifth and final year at Northwestern in 2018 and not enter the NFL Draft early.

If Thorson cannot start the season, the battle for the starting job would be among TJ Green, Aidan Smith and Andrew Marty. Incoming freshman Jason Whittaker figures to be a longshot at best, and snagging a graduate transfer is also a remote possibility.

Here are thumbnail sketches of the three primary candidates:

Green, a 6-foot-2, 190-pound redshirt freshman, is a walkon. He is the only one of the three to take a college snap, but has just one pass attempt and two rushes to his name so far. The son of former Indiana and NFL quarterback Trent Green, TJ comes from Missouri powerhouse Kansas City Rockhurst High School. He was named the Missouri Gatorade Player of the Year, USA Today Missouri Offensive Player of the Year and first-team all-state after throwing for nearly 2,700 yards and 33 touchdowns in 2014 and running for 300 yards and six more scores.

Despite the pedigree and numbers, however, Green didn’t draw a single offer from an FBS program. He picked up offers from FCS power North Dakota State and Ivy League Penn, and he had preferred walkon offers from Nebraska and Wake Forest.

Smith, also a redshirt freshman, was the lone scholarship quarterback in NU’s Class of 2016. A three-star prospect out of Ft. Wayne (Ind.) Carroll, Smith threw for 3,724 yards and 31 touchdowns and ran for 1,078 yards and 18 TDs in his high school career. He also won a 7-on-7 national championship with fellow Wildcat wide receiver Ben Skowronek. He chose Northwestern over offers from Boston College and five MAC schools.

He was a second-tier offer for Northwestern, after a couple other high-profile QB targets chose other schools. The 6-foot-2, 205-pounder is probably the best runner of the three QBs.

Marty, a true freshman, redshirted this season. At 6-foot-3 and 218 pounds, Marty is almost the size of Thorson already and has a prototypical quarterback's frame. A three-star prospect, Marty was also a second-tier Northwestern target, but he missed his sophomore season due to a broken leg, damaging his recruiting prospects. Marty had been a Miami (Ohio) commit before Northwestern came in with an offer.

Marty had an outstanding senior season at Cincinnati (Ohio) Wyoming, completing 64 percent of his passes for 3,202 yards and a school-record 43 touchdowns, against just four interceptions. He was named an all-state and all-region performer.

WildcatReport’s way-too-early take: If Thorson doesn't make it back in time for the opener, we think it will come down to a competition between Smith and Marty for the starting job. Smith has a year on Marty and has gotten more reps and film study. But Marty’s size and a stronger arm makes him our early favorite.

Thorson also has some options to consider, so everything is up in the air. He could make it back next year, or he could opt to sit out and either enter the NFL Draft or petition the NCAA for a sixth year for medical reasons (he already took a redshirt year, in 2014).

One thing is for sure: spring football at Northwestern figures to be much more interesting than usual this year.