The starting cornerback opposite of Tramaine Brock remains a mystery for the San Francisco 49ers.

The club rotated several defensive backs in the preseason. The process illustrated just how much depth is at the position.

Free-agent addition Shareece Wright along with 2014 draft picks – Kenneth Acker, Dontae Johnson and Keith Reaser – all saw first-team reps in training camp practices and preseason games.

The fluid nature of the competition appears to bode well for Eric Mangini's defense. San Francisco has several options for base and sub packages when they choose to announce their final choices prior to a Week 1 home matchup against the Minnesota Vikings.

Reaser is one of the many talented cornerbacks on the team's 53-man roster. The 24-year-old cornerback, however, is the only one of San Francisco's five cornerbacks who entered training camp without any preseason or regular season experience.

Last year's fifth-round draft pick out of Florida Atlantic missed his rookie campaign due to an ACL injury that he suffered in college. But after a year of vigorous rehabilitation work, Reaser now finds himself on the Week 1 roster and as an option to play a significant role on defense and perhaps special teams.

"It's exciting," Reaser said of his 2015 outlook. "Last year, I wasn't a part of the team like that because I wasn't playing. But now, I'm so much more excited for the games. I'm just ready to get started."

Reaser started in three preseason games for the 49ers. He totaled eight tackles and one pass breakup in his first NFL playing experience.

Reaser said he felt like the playing time helped him develop tremendously after a season away from the game.

"I think it went really well," the 6-foot, 190-pound defender said. "I feel like I improved each week. The game started slowing down little by little."

Reaser, the speedy cornerback, only wished he'd seen more activity in the four preseason contests.

"I feel like I didn't get a whole lot of action my way, in terms of teams throwing the ball at me," Reaser said. "But sometimes you get a lot and sometimes you get a little. That's the nature of playing cornerback."

Reaser's practice performances and game film will help state his case for winning a starting role on base and sub-package defenses. The young cornerback understands that the 49ers cornerbacks had a strong preseason as a whole and that key defensive roles will have been earned.

"Some plays didn't come my way, some plays did," Reaser said. "I feel like I played well and the other guys did, too. We'll see what happens with the (competition)."

After the 49ers beat the San Diego Chargers to finish 2-2 in the preseason, Jim Tomsula said the final decision on the team's cornerback roles would be shared in a timely manner.

"I feel like we're working hard," Reaser said of the cornerback group. "Everybody is together, working. I feel like we've got a lot to build on. Now we need to put together some wins, and we'll be good."