Fitzpatrick will replace Locker in the starting lineup after the latter was ruled out for the rest of the season with a Lisfranc injury suffered in Sunday's defeat to the Jacksonville Jaguars.
Fitzpatrick has already had a taste of the first team this season having come in for the matches against the Kansas City Chiefs and Seattle Seahawks, while Locker was recovering from a separate injury.
However, the 30-year-old failed to impress in those matches, throwing just one touchdown and four interceptions in two defeats for his side, and Walker believes that he has no choice but to improve this time around.
"Ryan knows he's got to play, we need him to step up. Ryan has no choice but to play good. I feel like he did a great job against the Jaguars. It's his opportunity to be the man now," Walker told reporters.
The Titans take on divisional rivals the Indianapolis Colts on Thursday as they look to close the gap on the AFC South leaders.