"I don't want to rule him out, but it's very doubtful that he's going to play," said Zorn, who added, in response to a reporter's question: "I would say 'very, very doubtful.'"
"He would play if he could, but he really is sort of mellowed out with this concussion," Zorn said. "... He needs to get rest. And I think it's a dangerous thing. I would never say, 'Come on, Clinton! You can do it!' That wouldn't be the right thing to try to push, and we're not going to do that. We're going to be cautious."
After meeting with Zorn, Portis left the Redskins' practice facility to see a doctor and have some medical tests. He didn't speak to reporters.
Portis ranks eighth in the NFC with 494 rushing yards this season for Washington (2-6), which has lost four games in a row. If Portis does sit out, Ladell Betts -- used mainly as a third-down back -- would make his first start since the 2006 regular-season finale.
The Redskins filled Horton's roster spot by re-signing running back Quinton Ganther, who was released last week.
Horton is Washington's third starter to go on IR this season, joining offensive linemen Chris Samuels (neck) and Randy Thomas (triceps). Other regulars expected to miss Sunday's game: Portis, tight end Chris Cooley (ankle), punter Hunter Smith (groin) and tackle Mike Williams (ankle). Zorn said Cooley will sit out for about another two weeks, then will be re-evaluated.
"Just trying to be smart about it," Campbell said. "Don't want to aggravate it."
"Right now, yeah, that's a 'ruled out' situation for us," Zorn said.
Copyright 2009 by The Associated Press