An undisclosed injury has forced Kelvin Gastelum (7-0 MMA, 2-0 UFC) out of a planned UFC on FOX 9 welterweight fight with fellow “Ultimate Fighter” winner Court McGee (16-3 MMA, 5-2 UFC).

MMAjunkie on Monday learned of the scratch from sources close to the fighter. A UFC official confirmed the change and that the promotion is actively seeking a replacement opponent.

Featuring a flyweight title fight between current champ Demetrious Johnson and challenger Joseph Benavidez, UFC on FOX 9 takes place Dec. 14 at Sleep Train Arena in Sacramento, Calif. The evening’s four-fight main card airs on FOX following prelims that on FOX Sports 1 and MMAjunkie.

Gastelum vs. McGee was expected to take place on the night’s preliminary lineup, though an official bout order hasn’t been revealed.

The 21-year-old Gastelum was a surprise winner of “The Ultimate Fighter 17,” upsetting heavily favored Uriah Hall in the season’s live finale and capturing the middleweight tournament title. He promptly dropped to 170 pounds and earned an impressive first-round submission finish of Brian Melancon in August.

A timetable for McGee’s return wasn’t immediately established.

Meanwhile, McGee also won a middleweight tournament, submitting Kris McCray to win “The Ultimate Fighter 11” title in 2010. McGee posted a 3-2 mark in the division before dropping to 170 pounds. Since the switch, he’s earned back-to-back decision wins in fights with Robert Whittaker and Josh Neer.

For more on UFC on FOX 9, stay tuned to the UFC Rumors section of the site.

(Pictured: Kelvin Gastelum)