Harris said Tuesday he hurt the patella tendon and hoped to have surgery soon as he heads to injured-reserve status. He hurt the same knee during the off-season.
"It is what it is, you know? It's something I've got to go through and must be done," Harris said.
More running backs out
Washington quarterback Robert Griffin III could be cleared to play in the regular season when he meets with his surgeon Thursday night.
"I know Dr. Andrews will sit down and evaluate Robert and probably just give us his recommendation," Shanahan said.
Even if Andrews gives the thumbs-up, the final decision on whether Griffin plays in the Sept. 9 opener rests with the coach. Shanahan has said that even if Griffin is medically ready, the coach could see something in practice that might give him pause.