Source: Brian Stann injured

UFC middleweight contender Brian Stann suffered a shoulder injury that has forced him to pull out of an Aug. 4 bout against former Bellator champion Hector Lombard, a source told ESPN.com on Wednesday.

The specific nature of Stann's injury was not revealed, but it is expected to sideline him for more than a month.

"No one is more disappointed than I," Stann said on Twitter. "(It) meant the world to headline a UFC."

Stann and Lombard were slated to fight in the five-round UFC on Fox 4 main event at Staples Center in Los Angeles. It would have marked Lombard's UFC debut.

Stann, a former WEC light heavyweight titleholder, is 4-1 a since moving to the 185-pound division. He has a professional mixed martial arts record of 12-4.

During his most recent fight on April 14, Stann looked impressive in knocking out fellow power-puncher Alessio Sakara in the first round.

Lombard, who signed with UFC in April, sports an eye-opening pro record of 31-2-1 with one no contest. He has not lost a fight since November 2006.

UFC has not yet released an official statement regarding Stann's injury, nor has the promotion indicated its immediate plans for Lombard.

With Stann unable to compete, UFC announced Wednesday night a new main event for the card.

Former light heavyweight champion Mauricio Rua will face Brandon Vera in the feature bout.

Rua originally was scheduled to meet Thiago Silva on July 21 at UFC 149 in Calgary, Alberta. But when Silva suffered an injury while training, the search to find another opponent for Rua began.

Vera, who was preparing for a bout against James Te Huna on July 11, got the offer to meet Rua instead.

UFC has not yet indicated whether it will seek a new opponent for Lombard or remove him from the card.