Broncos running back C.J. Anderson, who had a breakout performance in last week’s preseason game at San Francisco, was carted off the practice field Thursday with a right knee injury.

Tests on C.J. Anderson’s right knee revealed a sprain to the medial collateral ligament, according to an NFL source — and not a torn anterior cruciate ligament as was the fear when the running back went down in pain during practice Thursday.

Anderson is expected to be out four to six weeks. An undrafted rookie from California, Anderson had seemingly earned a spot on the 53-man roster as the fifth running back until his injury occurred.

Although Anderson is expected to recover, it might affect his spot on the 53-man roster. Besides their top four backs of Ronnie Hillman, Montee Ball, Knowshon Moreno and fullback Jacob Hester, the Broncos also have the ever-dependable Lance Ball.

“It’s awful, just awful,” Manning said of Anderson’s injury. “I don’t know the extent. It surely did not look good. You could hear him out there grimacing, which is not a sound any player likes to hear.”

The Broncos practiced with artificial crowd noise for the first time in training camp as they prepared to travel to Seattle, a stadium known for its raucous crowds.

Despite the distraction, Manning and the top offense looked sharp. Wide receivers Demaryius Thomas and Eric Decker made impressive catches under tight coverage.

The only hitch came at the end of the drill, when Manning was forced to call a timeout on a goal-line play. On the next play, his pass was intercepted by cornerback Champ Bailey.

Aside from Miller’s absence, the team’s injury list was the same as on Thursday. Running backs Knowshon Moreno (bruised knee) and Jeremiah Johnson (knee), wide receivers Quincy McDuffie (hamstring), Greg Orton (ankle) and Lamaar Thomas (concussion), safety Quinton Carter (knee), and tight end Joel Dreessen (knee) did not practice.

The team will fly to Seattle on Friday for its second preseason game on Saturday. Fox said on Wednesday he expected the starters to play the first half in Seattle.