ENGLEWOOD, Colo. -- Denver Broncos running backs Knowshon Moreno and Correll Buckhalter have both been injured on the Broncos' first day of training camp.

Moreno was carted off the field after hurting his right leg Sunday, and Buckhalter injured his left leg minutes later.

The Denver Post, citing unnamed sources, reported that Moreno suffered a serious right hamstring injury, while Buckhalter pulled his upper back. Moreno is expected to be out three weeks after an MRI exam revealed no significant tear to his hamstring, according to the Post.

According to the Post, Buckhalter was sent to a local hospital as a precaution but was released after his head, neck, back and spine were checked.

About an hour into the workout, Moreno caught a pass in 7-on-7 drills and turned upfield when his leg buckled. He grimaced as he hopped into the team's headquarters for tests.

Buckhalter hurt his leg about 10 minutes later during the same drill and limped off on his own.

Coach Josh McDaniels, who 48 hours earlier traded veteran running back J.J. Arrington to Philadelphia for special teams stud Joe Mays, said he didn't know the extent of their injuries.

"Certainly, you don't want to see two of your guys that you really count on at the same position come off the field on the first day of training camp," McDaniels said.

The Broncos' backfield now looks like this: waiver wire pickup Kolby Smith, undrafted rookie Toney Baker and second-year pros Lance Ball and Bruce Hall.

"One guy goes down, a guy moves up," Ball said. "Chance to get more reps, get on film and impress the coaches."

Information from The Associated Press was used in this report.