Starting exhibitions while Griffin comes back from offseason knee surgery, Cousins left Monday night's 24-13 win over Pittsburgh early in the second quarter.
He suffered a mid-foot sprain, but said initial X-rays came back negative. And, for now, a Lisfranc injury also was ruled out.
Meanwhile, nose tackle Barry Cofield suffered a fracture in his right hand but is optimistic about playing in the season opener.
Cousins, who was wearing a boot on his right foot and left on crutches, will undergo an MRI Tuesday to determine the extent of his injury. With starter Griffin still not certain to return for the opener, the Redskins can't afford to have Cousins miss any time. They list as their No. 3 quarterback Rex Grossman, who entered to play quarterback for Washington and went 10-for-16 for 133 yards with one touchdown and one interception.
Cousins hurt his foot on a second-quarter scramble off a bootleg. He was dragged down along the sideline by linebacker Lawrence Timmons, but returned to the huddle. He then sat down on the grass.
On the second of two consecutive plays on which he ran the ball, he was examined on the field and on the sideline, then walked to the locker room.
"I had to self-evaluate how much if I had to change directions and be an athlete, could I do that," he said. "I decided preseason game No. 2, with a long season to go, now's not the time to be a hero."
Cofield said doctors told him his break was clean and would not need surgery. He said he wasn't sure if he'd play in the final two preseason games.
"Most guys would be in the hospital in intensive care," Redskins coach Mike Shanahan said. "But he's able to talk to you guys so it gives you an idea of how tough he is. He'll be able to play through it."
Information from ESPN.com Redskins reporter John Keim and The Associated Press was used in this report.