In their past two games, the 49ers have played without their starting nose tackle, two All-Pro linebackers and their nickel cornerback.

But who needs them, anyway?

In a testament to its depth, shorthanded San Francisco has dominated using a defensive lineup dotted with reserves. In blowout wins over the Rams (35-11) and Texans (34-3), the 49ers have allowed an average of 250.5 yards a game and one touchdown, and they've forced six turnovers, three more than they collected in their first three games.

In Sunday night's romp over Houston, the defense was headlined by two backups who are former undrafted free agents from non-Division I programs.

No. 3 cornerback Tramaine Brock, subbing for sidelined Nnamdi Asomugha, had two interceptions, including one he returned for a tone-setting touchdown on the game's third play. It was Brock's first touchdown since he returned two picks for scores as a senior at Belhaven (Miss.) College in 2009.

Meanwhile, inside linebacker Michael Wilhoite made his second straight start in place of Patrick Willis and had 11 tackles, including three for losses, a team high. The last time Wilhoite had double digits in tackles, he was playing at Division II Washburn University in Topeka, Kan., in 2010.

"Guys get their opportunity and step up and play well," 49ers head coach Jim Harbaugh said. "That bodes really well for the ballclub."

Something else that bodes well for the 49ers: It appears they won't have to lean on another super sub.

Brant Ward/The Chronicle

On Tuesday, Harbaugh said the biceps injury that sidelined Ray McDonald against the Texans wasn't serious. The defensive tackle exited following a tap-down sack of Matt Schaub in the third quarter. McDonald said he felt a minor pain, like a "bee sting," as he was fending off a blocker en route to the sack.

McDonald underwent an MRI exam Monday and the results were encouraging.

"We dodged a bullet there," Harbaugh said on KNBR. "It looks like Ray might have walked one off."

In McDonald's absence, Tony Jerod-Eddie, another former undrafted free agent, became the latest backup to come up big. In 29 snaps, Jerod-Eddie had a tackle, a quarterback hurry and an interception, the first pick by a 49ers defensive lineman since 2010.

Jerod-Eddie, Brock and Wilhoite were with the 49ers last year, but the rest of the recent defensive holes have been filled by offseason acquisitions.

At nose tackle, Glenn Dorsey, a free-agent signing, has assumed the starting spot vacated when Ian Williams was lost for the season with a broken ankle in Week 2. Since Williams was injured in the first half of a 29-3 loss at Seattle, Dorsey has collected two sacks, which is tied for third on the team and is the most among defensive linemen.

With outside linebacker Aldon Smith in a treatment facility, Dan Skuta, a free-agent signing, and rookie Corey Lemonier, a third-round pick, have shared snaps and combined for two quarterback hits and eight hurries the past two weeks, according to Pro Football Focus. Skuta is another contributor with a modest pedigree. An alum of Division II Grand Valley State, he started four games in his first four seasons in Cincinnati, where he was known mainly for his special-teams play.

Despite the solid performances of their backups, the 49ers are obviously eager to welcome back Smith and Willis, who was close to playing Sunday after sustaining a groin injury in Week 3.

It appears Asomugha, however, might have lost his spot as the No. 3 cornerback. Before his two-interception performance, Brock batted away a ball that safety Donte Whitner intercepted in Week 4, a play defensive coordinator Vic Fangio termed the biggest of the game.

Both Brock and Asomugha have performed well this season in place of yet another sidelined defensive player. Chris Culliver, the No. 3 cornerback the previous two years, is out for the season with a torn ACL.

"Really pleased with the way Tramaine's ascending," Harbaugh said. "When you play like that, you're going to continue to play. So, I think there's going to be more opportunity for him."