Diego Sanchez has been forced to withdraw from a highly anticipated welterweight showdown with Thiago Alves at UFC 90.

Sanchez representative Jeff Clark told Sherdog.com that the injury was sustained in a training session last week, and what was originally thought to be a ruptured spleen turned out to be damage to his ribs, possibly to an intercostal muscle or cartilage.

The Oct. 25 card scheduled for the Allstate Arena in Rosemont, Ill., will not suffer greatly, however. Fellow welterweight standout Josh Koscheck has agreed to step in for his injured fellow alum from the inaugural season of “The Ultimate Fighter.” While the bout agreement is yet to be signed, parties close to Koscheck and Alves have confirmed to Sherdog.com that the fight has been agreed to.

Sanchez (19-2-0), who dropped two consecutive fights in 2007, had resurrected his title hopes with a pair of victories this year and would have surely been considered a top contender had he been able to derail the streak Alves has been riding.

The popular Alves (15-3-0) has ascended to the top of the welterweight contender’s ranks with consecutive victories over former champion Matt Hughes and Karo Parisyan. He is in the midst of a six-fight winning streak with his last loss coming at the hands of Koscheck’s teammate Jon Fitch in June 2006.

Koscheck (11-2-0), a collegiate wrestling champion, will look to extend his two-fight winning streak and regain the momentum he took into his August 2007 bout with now-UFC champion Georges St. Pierre. Koscheck, viewed by many as an eventual contender will have a chance to leapfrog Sanchez in his quest for a title shot as well as redemption against the last man to beat him.