McNair suffered a partial dislocation in his nonthrowing shoulder late in Sunday's 21-7 loss to the Cincinnati Bengals, coach Brian Billick announced on his weekly radio show last night.
Boller, who was a three-year starter before the Ravens traded for McNair in June 2006, is 2-1 as the starter this season. In his three starts, Boller has averaged 186.7 passing yards, throwing three touchdown passes and one interception.
"Obviously, Kyle is cranked up and ready to go," Billick said.
McNair's injury raised eyebrows because it comes at a point when the Ravens might have been ready to bench the well-respected, four-time Pro Bowl quarterback.
In six starts this season, McNair has struggled to a 2-4 record, throwing two touchdown passes and committing 11 turnovers (four interceptions, seven fumbles).
McNair was pulled from Sunday's loss with 7:33 left in the game after he failed to produce a scoring drive against the Bengals defense, which had allowed 30.5 points a game. In the locker room, a subdued McNair indicated he had reached the "lowest point" of his career and said he would understand if the Ravens decided to bench him and start Boller.
A day later, the Ravens announced that McNair suddenly had a shoulder injury. Billick said last night that McNair was injured on his last hit.
According to Billick, the shoulder keeps "popping in and out" of its joint, which will cause McNair to be sidelined a "minimum" of two to three weeks before he can begin practicing.
"Initially, it was one of those things where there's a numbness to it and then you wake up in morning and it's, 'Oh my God, what's going on here?'" Billick said.
Over the past four seasons, McNair has missed 13 starts. He has sat out three starts this season with groin and back injuries.
According to Billick's timetable, Boller would start against the Browns and at the San Diego Chargers. The earliest McNair would return - and that's presuming the Ravens would be willing to start him again - would be Dec. 3 against the New England Patriots.
Boller, a 2003 first-round pick by the Ravens, brings a different style than McNair to the Ravens offense, which has dropped to No. 23 overall and No. 28 in scoring.
There's a better chance of producing big plays with Boller because he looks downfield more often than McNair and has a stronger arm. Boller also has more mobility and can create more plays with his legs.
With Boller becoming the starter, rookie fifth-round pick Troy Smith now becomes the backup. Last season's Heisman Trophy winner has not taken a snap during the regular season.
"That's going to be a fun challenge for Troy, because now he's been around enough," Billick said. "The focus and attention that he brings to [preparing] is heightened."
Because Smith is the second quarterback (the No. 3 essentially can't play the first three quarters unless the top two quarterbacks are injured), Billick speculated that Smith could be put into some specialty packages to take advantage of his athleticism.
"You don't want to tip your hand, but there are some things that you could do with Troy in that capacity because of his abilities to run the ball and do some of those things," Billick said.
At this point, the Ravens aren't considering signing another quarterback. But if McNair's injury lingers, the team might think about adding one for depth.
Said Billick: "Quite frankly, there's not a lot out there for us to consider."