Last spring as the 49er faithful gathered around their tvs to watch the draft I can imagine a giant "huh?" go up when Aldon Smith was selected 7th overall. He wasn't a highly touted draft pick when the 49ers made him their first round selection. Then the 49ers passed on a CB in the second round to select Chris Culliver in the third. Culliver was another no-name draft pick.
Both of them have turned out far better than anyone could have expected and both are possible Defensive Rookie of the Year candidates.
Aldon Smith
Smith is fifth in the league in sacks at 5.5. You might recognize the names ahead of him. Jared Allen, Demarcus Ware, Jason Babin, Jason Pierre-Paul. What's truly amazing is that the's done this despite not being a starter. He's taken all of 201 snaps this season. The next lowest amount of snaps is Jason Babin at 282. It's not unreasonable to assume that with adequate playing time Aldon could end up with double digit sacks. In fact he's on pace for 14.5 sacks on the season which would tie the all-time rookie sack record which was set by Jevon "The Freak" Kearse in 1999. I don't think it's unreasonable to think that Aldon Smith could beat that record.
He also has a safety and a forced fumble.
Chris Culliver
Culliver is another 49er defensive player who is not a full-time starter though I expect that to change. In fact he didn't get any significant playing time until the Tampa Bay game where he played 29 snaps, then the Detroit game he played 69. In that time he's managed to get 8 tackles, an INT and 4 PD. If he plays this well the remainder of the year he could end up with over 20 passes defended and 6 INTs. The odds against him are slim--a DB hasn't won Defensive Rookie of the Year since Dale Carter did it in 1992.
Other candidates:
Ryan Kerrigan (Washington Redskins)
Kerrigan has also been off to an impressive start. He has two sacks, three passes defensed, two forced fumbles, one interception and a touchdown. He's also played significantly more snaps than Aldon Smith has, playing in 330 of his team's 330 defensive snaps.
Von Miller (Denver Broncos)
Von Miller got off to a great start early on. He has 5 sacks, 2 FF, and 2 PD. It's not unreasonable to except that he would end up with double digit sack numbers as well, other than the fact that he was benched in last week's game because he lacks situational awareness and gives up huge runs because he's so intent on getting to the quarterback that he leaves his gap completely undefended. If he can fix that situational awareness he could be a factor in rookie of the year honors.