Last year, it was a chest injury that put strong safety Antoine Bethea on the sideline for the final eight games of the season. This time, it's a torn biceps in his right arm that ended free safety Eric Reid's season with six games left in the year.
"Going through last year, it was my rookie season and it just prepared me again for this year," Tartt said. "So I always had that mindset of just preparing as if I'm starting. I've been doing that since the season started so it's no big deal."
While appearing in 15 games with those eight starts a year ago, Tartt posted 65 tackles, two sacks, a forced fumble and an interception. Along the way, Tartt said he was able to learn from Reid, which helped him become a better player than when he began the season.
This time around, Tartt is expecting even more of himself as he steps in for Reid. Although the 49ers list a free and strong safety, they actually use them interchangeably. That should help limit potential mental miscues for Tartt.
"I expect to be better," Tartt said. "In everything, in every phase of the game. My overall awareness, just seeing plays better, reacting faster to the ball. Last year I got to play with E-Reid and that was a good opportunity to pick his brain a little bit and this year I have A.B., so I get to pick his brain."
Of course, while Tartt is technically filling in for Reid, the Niners won't ask him to take on the workload that Reid was handling. Reid was the most versatile player on the defense, playing both safety spots and in the nickel and even occasionally lining up as an edge rusher. He's even been a dime linebacker over the past few weeks.
Even if Tartt has the athletic skills to do those many jobs, the Niners won't ask him to do it, instead hoping that he can focus on just evolving as a safety.
“Tartt's going to obviously step into the safety role, but other guys in other positions are going to have to step up and do some of the jobs that he was able to do for us in passing downs,” defensive coordinator Jim O'Neil said.
For example, defensive back Dontae Johnson figures to be the third safety and the Niners could move some things around when and if they choose to use the three safety looks they had with Tartt, Reid and Bethea.
Reid got many opportunities to play near the line of scrimmage but the 6-foot-1, 221-pound Tartt has proved better in coverage than defending the run in the early parts of his career. The 49ers used Tartt almost exclusively to cover Patriots tight end Martellus Bennett last week and Bennett finished with just one catch for 14 yards, a play that actually came with Reid covering him.
“He's got great range and ball skills," O'Neil said. "He's got a knack for reading the quarterback. I mean, you guys are familiar with him. He's played a lot of football. He's played a lot of football for us this year and he filled in and did a great job last year when Antoine Bethea went out. So, he's a good football player.”
Although the circumstances of Tartt's latest promotion aren't ideal considering the cost, the silver lining for the 49ers could be the opportunity to get a better idea of just what they have in Tartt going forward.
Reid and Bethea will be free agents after the 2017 season, which means an evaluation on Tartt isn't quite urgent but is something that needs to be in their crosshairs sometime in the near future. Before Reid's injury, Tartt had played about a third of the snaps and some had suggested he'd been underutilized by the Niners.
Even if Tartt's role won't be as expansive as Reid's was, he's at least familiar with what it takes to step in for a key veteran on the back end.
"It's very similar," Tartt said. "For me, I have just got to run the plays they call and just do what I do."