SAN FRANCISCO (AP) -- When Steve McNair went down with a back injury one week into the season, the Tennessee Titans had Neil O'Donnell to fall back on at quarterback.

The 10th-year pro and onetime starter for Pittsburgh, Cincinnati, and the New York Jets has stepped right in, leading the Titans (3-0) to consecutive wins over Cleveland and Jacksonville. Tennessee has chance to come out of Sunday's game at San Francisco (2-1) with the franchise's first 4-0 start.

And they face a 49ers team that is without Steve Young, sidelined by a concussion suffered midway through last Monday night's 24-10 win at Arizona. Jeff Garcia, who spent five years in the Canadian Football League, will make his first NFL start in Young's place.

Tennessee's Eddie George has struggled to get his running game going, but has nevertheless scored four touchdowns in the first three games. George said the absence of Young and his replacement by an NFL neophyte does not make the encounter with the 49ers any less challenging.

"I expect them to raise their level of play," George said. "Their defense is definitely going to come out to play, raise it up a notch, same with special teams. The 49ers are still the 49ers."

Titans coach Jeff Fisher noted the 49ers have an imposing record at home -- they've won 18 straight -- and have a history of success without their starting quarterback. Indeed, in eight game in which Young was absent due to injury since 1996, the 49ers have gone 6-2 with Elvis Grbac, Jim Druckenmiller and Ty Detmer at quarterback.

"We're very familiar with how the 49ers have done in games Steve Young has not played," Fisher said. "Whether it's Druckenmiller, or Detmer or Grbac, the 49ers have found a way to win when Steve has not been there. I think it's a tribute to the system, but I also feel like Jeff Garcia has the potential to be a very good quarterback in this league."

The Titans, though, have also won without McNair, who is expected to be sidelined for five more weeks after undergoing back surgery.

"That's because that's what we had to do," George said. "We had to believe in Neil. And I think we did, during training camp and during the offseason, that, hey, if something happened to Steve, and he goes down, that Neil will come in and we wouldn't miss a beat. And it's been just like that thus far."

It's also helped that the former Oilers finally were able to settle into permanent quarters and a new stadium this season, four years after their move from Houston.

"Everything's stable now and definitely, you can feel it around here," George said.

San Francisco's week leading up to the game has been more chaotic. There's been concern, by teammates and coaches alike, about Young's football future after suffering his fourth concussion in three years. Young has said he has no plans to retire and expects to return in a week or two following another exam by his neurologist.

Unlike Druckenmiller, who won his only start for San Francisco as a rookie in 1997 starting in place of a concussed Young, Garcia does have professional experience. He was a onetime teammate of Doug Flutie in the CFL and last year led the Calgary Stampeders to the Grey Cup championship.

And he has the support of 49ers general manager Bill Walsh, who handpicked Garcia to come to San Francisco as Young's backup.

"I feel so fortunate because of my Canadian experience," said Garcia, who came on for Young in the second half of the Arizona game.

"I feel I've been through pressure-type situations. I've been in a championship game. I've dealt with having to come back with less than a minute left. I've dealt with big crowds, loud noise, tough situations. I think my maturity through the CFL is definitely going to help me with my experience on the field on Sunday."

And, Garcia said, he'll have the benefit of working with top playmakers in a well-tested offensive system.

"I think the main thing is to go out there and play within myself, just allow the system to work for me, and allow myself to get the ball to one of the best receiving corps in the league," Garcia said.

He'll also have an expanded play selection.

After Young went down in the Arizona game, San Francisco's play-calling turned conservative because Garcia had little preparation. He's had a week to get ready and has taken virtually all of the practice snaps.

"We won't go into the game thinking conservatively," 49ers offensive coordinator Marty Mornhinweg said. "Now we'll certainly give Jeff some plays he feels comfortable with. But we're not 17 points up at halftime like we were in Arizona. We're going into this game 0-0."

TENNESSEE Season schedule and results

SAN FRANCISCO Season schedule and results

September 12 Cincinnati W 36-35 September 19 Cleveland W 26-9 September 26 @Jacksonville W 20-19 October 3 @San Francisco 4:15pm ET October 10 Baltimore 4:15pm ET October 17 @New Orleans 1:00pm ET October 31 St. Louis 1:00pm ET November 7 @Miami 8:20pm ET November 14 @Cincinnati 1:00pm ET November 21 Pittsburgh 1:00pm ET November 28 @Cleveland 1:00pm ET December 5 @Baltimore 1:00pm ET December 9 Oakland 8:20pm ET December 19 Atlanta 1:00pm ET December 26 Jacksonville 1:00pm ET January 2 @Pittsburgh 4:15pm ET

September 12 @Jacksonville L 3-41 September 19 New Orleans W 28-21 September 27 @Arizona W 24-10 October 3 Tennessee 4:15pm ET October 10 @St. Louis 1:00pm ET October 17 Carolina 4:15pm ET October 24 @Minnesota 1:00pm ET November 7 Pittsburgh 4:15pm ET November 14 @New Orleans 1:00pm ET November 21 St. Louis 4:15pm ET November 29 Green Bay 9:00pm ET December 5 @Cincinnati 1:00pm ET December 12 Atlanta 4:15pm ET December 18 @Carolina 4:15pm ET December 26 Washington 8:20pm ET January 3 @Atlanta 9:00pm ET

Records source: STATS, Inc.

Copyright 1999 STATS, Inc. Commercial distribution without the express written consent of STATS is prohibited.