The highly-anticipated World Extreme Cagefighting arrival of former UFC lightweight champion Jens Pulver (Pictures) (21-8-1) has been indefinitely put on hold, sources close to the Miletich champ told Sherdog.com Monday evening, due to a knee injury suffered by the popular fighter last Friday.

Pulver was scheduled to face promising prospect Cub Swanson (Pictures) (11-1-0) on Sept. 5 at the Hard Rock Hotel & Casino in Las Vegas. It would have been Pulver's first fight in the 145-pound featherweight division, a class much more suited to the Iowan, who dropped his last outing to B.J. Penn (Pictures) in June.

According to the source, an MRI is scheduled for later this week. Those results will determine whether Pulver will face surgery or physical therapy.