The 29-year-old, Washington's primary running back since 2004, had already missed five games with a torn left groin before suffering an abdominal injury during Sunday's 19-16 win over the Tennessee Titans.
It is the second season in a row for Portis to be placed on injured reserve, and the third time in the last five years.
In 2006, he was sidelined by hand and shoulder injuries while last year he was ruled out after being severely concussed.
"(It) really is disappointing because he has worked so hard to get to where he was at," Shanahan said of a player who needs just 77 yards to reach 10,000 for his career.
"He's encouraging all the other players and he's a big part of this football team. You hate to see a person work that hard and not get an opportunity to finish the season."
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)