The Redskins plan to start Grossman on Sunday against the Dallas Cowboys, benching a six-time Pro Bowl quarterback billed as the next John Elway when he came to the franchise in April.
Shanahan said he informed McNabb of the move Thursday after practice, adding he wasn't sure that McNabb would be on the roster next year. The move was made because Shanahan wants to evaluate the other quarterbacks now that the Redskins have been eliminated from the playoffs.
McNabb will drop to third string for the final two games, with John Beck serving as backup.
The 34-year-old McNabb is struggling through his worst season since he was a rookie in 1999, having thrown a career-high 15 interceptions and ranking 25th in the NFL with a 77.1 rating.
McNabb was infamously benched for Grossman in the final two minutes with the game on the line against Detroit in October, a stunning decision made even more bizarre by Shanahan's mangled explanations that followed.
The coach first said he felt Grossman had a better grasp of the team's two-minute offense, then said McNabb lacked the "cardiovascular endurance" to run a fast-paced drill because of nagging hamstring injuries. Offensive coordinator Kyle Shanahan said McNabb had been tipped in advance that the team might go with Grossman; McNabb said he hadn't.
Before the next game, the Redskins (5-8) gave a McNabb a five-year contract extension nominally worth $78 million - but the only thing it guaranteed was that McNabb would receive an extra $3.5 million this season. The deal contains a clause that allows the Redskins to cut McNabb before next season with no further financial obligation.
This week, the situation began again to take more strange turns.
McNabb said his communication the coaching staff had improved since the benching; neither of the Shanahans would agree with that. McNabb said he would expect to know by Wednesday if he weren't starting, calling it a matter of "professionalism."
On Thursday, Kyle Shanahan seemed to go out of his way to avoid praising McNabb while meeting with reporters, but he implied McNabb was still starter when he said the preparation for this week was "no different" from last week as far as the quarterbacks were concerned.
This certainly wasn't the vision the Redskins presented when they traded two draft picks - a second-round choice this year and a third- or fourth-rounder next year - to Philadelphia for McNabb. It seemed curious the Eagles would trade a quarterback to a division rival unless they felt his best days were behind him, but the Redskins couldn't stop gushing at a news conference two days later, noting that Elway was roughly the same age as McNabb when he led Mike Shanahan's teams to two Super Bowl titles with Denver in the 1990s.
"People were saying John Elway should retire," Mike Shanahan said at the time, "until he won the Super Bowl."
Grossman was signed by the Redskins in the offseason because he was familiar with the team's new offense and would be able to help teach it to McNabb. Grossman was a backup last year in Houston, where Kyle Shanahan was the offensive coordinator.
In his only appearance this year - relieving McNabb in the loss to Detroit - Grossman fumbled while being sacked on his first play, and the ball was returned for a touchdown. He finished the game, going 4 for 7 for 44 yards.