Jim Harbaugh made it clear last November: a player who get injured certainly can (and will) lose his job if the next man on the depth chart outshines what the previous guy did while healthy. Alex Smith got knocked out against the Rams. In stepped Colin Kaepernick, who made enough plays against Chicago to take the job away from Smith.

Smith’s response: “I feel like the only thing I did to lose my job was get a concussion.”

Tramaine Brock 49ers preseasonSmith was playing as efficiently as anyone in the NFL when he got hurt. Unlike Smith, Nnamdi Asomugha was not performing like one of the best 15 players at his position before missing Thursday’s game against the Rams with a right knee injury. In fact, Pro Football Focus has Asomugha as one of the 15 worst players at his position over the first four weeks of the season (90th out of 101 cornerbacks).

In stepped Tramaine Brock, who performed quite well in St. Louis. Sam Bradford’s rating was 52.1 when throwing passes in Brock’s direction. Brock had two passes defensed, with one leading to an interception for Donte Whitner. In the first three games of the season, Asomugha only had one PD and four missed tackles. He also looked lost at times against the run.

Based on what we’ve seen thus far from Brock, “ball skills” aren’t a problem. Brock tipped and intercepted several passes throughout training camp, and came into this season with two INT and five PD in limited time. However, Brock isn’t as big as Asomugha and he went up against Bradford, who’s closer to Blaine Gabbert, Brandon Weeden and Matt Flynn on the quarterbacking scale than players like Aaron Rodgers, Russell Wilson and Andrew Luck.

Someone will almost surely ask Vic Fangio this week whether Brock or Asomugha will be the No. 3 cornerback against Houston. Unless Brock sustained an injury and Asomugha goes into Sunday fully healed, Brock likely will get the nod (although Fangio may decline to answer the question as early as Thursday, when he’ll probably head to the podium).

If Brock does play and performs well against the Texans, look for him to keep the job for several weeks. That scenario almost seems likely, since Keshawn Martin and DeVier Posey (the receivers behind Andre Johnson and DeAndre Hopkins) have combined for eight receptions and 107 yards so far this season.

Plus, Schaub has thrown a pick-6 in each of the last three games — it would help Brock’s cause if he could run that streak to four.