Their secondary in particular.
The St. Louis Rams come to town this week, and that means another stringent test for San Francisco's defensive backs, who were torched for 394 yards passing by quarterback Kurt Warner and the NFL's No. 1-ranked passing attack in Week 3 of last season.
"It was certainly baptism by fire," 49ers coach Steve Mariucci said. "Ready or not, we put them out there. We got in there and we went through it. It was a growing experience, maturing quickly. They will be more veteran-like for having played, much more ready to do this."
The Niners started rookies Ahmed Plummer and Jason Webster at cornerback last season, and rookies John Keith and Ronnie Heard also started games at safety. Plummer and Webster will be in the starting lineup again Sunday, this time with a much better idea of what will be coming at them.
The 49ers finished 29th in the NFL against the pass last year, but started this season with a big turnaround, allowing the Atlanta Falcons only 107 yards through the air in San Francisco's season-opening 16-13 overtime victory on Sept. 9.
That has the Niners ranked second in the league against the pass and fifth overall on defense entering Sunday's games. But everybody knows that could change in a hurry against the Rams.
"The Rams like to pass the ball. That's no secret. Everybody knows they're at the top as far as their offense goes," Plummer said. "We in the secondary have to take care of the pass. If you can play with them, you should be able to play with anybody in the league."