Tyrone Spong is having a hard time making his big MMA return.

The world-class champion kickboxer is injured and out of his co-main event fight with Jake Heun at World Series of Fighting 26 on Dec. 18 in Las Vegas, WSOF president Ray Sefo confirmed with MMA Fighting's Ariel Helwani. The MMA Kings was the first to report the news on Twitter.

In Spong's place will be Clinton Williams, according to Sefo. Williams (7-2), a well-muscled heavyweight, will be making his WSOF debut. The 31-year-old has won four of his last five fights. Williams is coming off a third-round submission of Jared Hatch last month at SteelFist Fight Night 38.

Heun, though, tweeted Tuesday that he had yet to sign a contract to fight Williams.

Spong (2-0) has not fought in MMA since August 2013. The three-sport athlete (he also does boxing) broke his leg last year during a GLORY kickboxing match against Gokhan Saki, but was supposed to return to WSOF over the summer. He pulled out with an injury then, too.

Heun (8-4) is coming off a third-round submission win against Davin Clark at WSOF 22 in August. The 28-year-old, who trains out of American Top Team, has won three in a row.

WSOF 26 is headlined by a featherweight title fight between Lance Palmer and Alexandre Almeida. It's unclear if Heun vs. Williams will be the co-main event. The event takes place at The Chelsea at The Cosmopolitan Las Vegas.

(This story was updated with information from Heun.)