Tennessee lost two straight games with Fitzpatrick at the helm, which may have hastened Locker's original prognosis of being out 4-6 weeks.
Locker was cleared and returned to practice Wednesday, throwing passes while sporting a bulky knee brace and noticeable limp. The Titans would certainly prefer to have their dynamic quarterback play against a 49ers defense that's forced 10 turnovers in its last three games. Fitzpatrick, meanwhile, has thrown two interceptions in each of the last two weeks.
The 49ers are preparing for Locker, just in case.
"They may use some of the read-option game with him. So, we've got to be ready for that. So, we are preparing for both quarterbacks," defensive coordinator Vic Fangio said Thursday.
Locker started 11 games for the Titans in 2012, throwing for 2,176 yards, 10 touchdowns and 11 interceptions. But prior to his injuries, he was emerging in his first full season as starter. The Washington alum has six touchdown passes and has yet to throw a pick.
"You're never in a position of knowing until you know, until they're ruled out," Jim Harbaugh said.
One of Locker's best attributes is his mobility, which could be hampered by his knee and hip issues Sunday, should he get the nod.
"He's like a running back with his ability to run the football," Harbaugh said. "And I've seen it up close, been defeated by it. he's evolved, grown. He's always been an extremely tough competitor. That's why you, knowing the competitor he is, if there's any possible way that he would play he'll be out there."
Locker and Harbaugh competed against one another in the Pac-10 from 2007 to 2010. Locker went 1-3 against Stanford with his lone victory coming in his freshman season of 2007. He led his three-win Washington team to a 27-9 victory while scampering for a pair of touchdown runs.
Sunday's game is an important one for both teams. A win for Tennessee could put them in a tie for first place in the AFC South while the 4-2 Colts host Peyton Manning and the undefeated Broncos.
For San Francisco, they remain just a game back of first place Seahawks (5-1) in the NFC West, who travel to Arizona for a divisional matchup Thursday night. Should Seattle fall, the 49ers would be in a position to tie them atop the standings with a win.
Before leaving the game with his injuries, Locker completed 18-24 passes against the Jets, which would have set his career high for completion percentage in a single game.
Fangio and the 49ers faced Fitzpatrick in 2012 when they handled the Buffalo Bills 45-3 in Week 5. Fitzpatrick threw for 126 yards on 16-26 passing and an interception.
"They're fortunate to have him as a backup. A guy that's played a lot in the league," Fangio said. "He's played winning football in his past. He's a very elusive guy. He's a scrambler. He's confident. He can make all of the throws. He's got good arm strength. So, I think they feel pretty good about him being in there."
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