The source said McKissic suffered a Jones fracture, which is on the outside of his foot.
McKissic has been competing with C.J. Prosise and Mike Davis for a roster spot in Seattle's crowded backfield, behind Chris Carson and first-round pick Rashaad Penny. McKissic was considered to have a good shot at making the team because of his ability to also play receiver and return kicks.
Penny returned to practice Tuesday after having surgery last week on a broken finger. Coach Pete Carroll reiterated that Penny will be ready for Seattle's Sept. 9 opener against the Broncos in Denver and said definitively that wide receiver Doug Baldwin (knee) also will be ready for that game.
Carroll said the status of defensive end Dion Jordan is still uncertain.
"We'll hold out hope that maybe there's a chance for the opener," Carroll said of Jordan, who is on the physically unable to perform list because of a leg injury. "We'll see. We don't know that yet. That's probably ahead of the schedule that the docs would say, but I know what he's talking like. He's going for it, and then we'll make good decisions with the information when we get it."
The 25-year-old McKissic took over as Seattle's third-down back last season following an injury to Prosise and finished with 453 combined rushing and receiving yards. His 30-yard touchdown run in Week 4 was Seattle's only rushing touchdown by a running back.