SANTA CLARA, Calif. -- The book on Iowa offensive linemen: They're going to be better run blockers than pass protectors when they first arrive in the NFL.

That's what the San Francisco 49ers were expecting after taking Connor Colby late in the seventh round in April. Instead, he showed up more accomplished in the passing game, winning his one-on-one battles in training camp and winning over the coaching staff in the process.

"He has been a tough guy to get around," 49ers head coach Kyle Shanahan said before practice on Wednesday.

It's why Colby edged out the more experienced Nick Zakelj to land a roster spot in August and why he was the first offensive lineman off the bench when left guard Ben Bartch went down with a high-ankle sprain Sunday in New Orleans. Colby played 61 snaps -- 38 in pass protection -- and came away with a clean sheet: zero pressures allowed.

Shanahan was not surprised.

"That's why he made our team," he said. "He beat out some good players, some guys who have been in this league before. So I think in order to do that, we've got to feel pretty high on him."

Colby is one of several rookies the 49ers are very high on who are making contributions. Three started on defense in the win over the Saints -- defensive end Mykel Williams, slot cornerback Upton Stout and safety Marques Sigle -- while defensive tackle Alfred Collins doubled his playing time from Week 1 and had his first quarterback hit.

Williams set the edge well and finished with five tackles, two for a loss of yards.

Williams and Collins "did some good things in Game 1, but we saw even better in Game 2," Shanahan said. "Alfred got out there more, flashed a bunch. I see both of those guys just working the right way, made of the right stuff."

Stout had his first sack, while Sigle had five tackles. Fourth-round defensive tackle CJ West played 13 snaps. Third-round linebacker Nick Martin has been inactive in the first two games, in part because Dee Winters is playing so well. (On offense, both receiver Jordan Watkins and running back Jordan James were battling injuries but are now healthy.)

Selected in the fifth round out of Kansas State, Sigle was able to win a starting job despite missing much of the team's offseason workouts with a sports hernia.

"We loved his ability with his speed and the way he hits and everything," Shanahan said. "But, even when he was out, (seeing) just how attentive he was in meetings, the way he'd ask questions, once he got his opportunity in training camp, it seemed like he had been working throughout (offseason workouts)."

Christian McCaffrey has been impressed.

"It's been fun watching him because he's somebody that you wouldn't know is a rookie," the running back said. "He's quiet, he's poised, but he plays with so much confidence and energy, especially at that safety position. One thing I always look at is people who don't hesitate."

Shanahan loves more than just the 5-foot-11, 199-pound Sigle's mental approach.

"He's got the speed, and he's got the courage to hit, and his body's built that way that can make him a pretty good player," Shanahan said.

When it comes to bodies, Colby doesn't have the stocky frame of a traditional guard. His silhouette, at least, suggests an offensive tackle.

"I didn't think he was a guard when he first came. I thought he'd play tackle," left tackle Trent Williams said. "He's what I would call a 'tweener -- in a good way. He has the size and length to play tackle but has the strength to play guard."

That strength was evident right away, both in the team's weight room and when he was asked to take on the 49ers' defensive tackles. Center Jake Brendel called Colby's power "applicable strength."

"He moves a lot of weight in the weight room," Brendel said. "That's not always 100 percent what this game is. But it can definitely help you. And he also shows that strength if you watch the one-on-one reps and stuff. He has a decent anchor and some decent hands, and he can lock people out, which is always a question for a younger guy. Does he have the ability to stop a bull rush, to stop a guy from pushing the pocket back?"

Colby also took a lot of repetitions during the spring and summer. Like Dominick Puni last year, he didn't miss any OTA and training camp practices, which allowed him to get progressively better and gain the coaches' trust. When Puni went down with a knee injury this preseason, Colby replaced him at right guard. And with Bartch now on injured reserve, Colby will make his first NFL start Sunday at left guard against the Arizona Cardinals.

"We all have a ton of confidence in the kid," Brendel said. "It kind of looks like he's played a lot more ball than what a normal rookie has played."

"I think it's his demeanor," Williams added. "Just the way he handles himself, the way you come to work, him being coachable, his willingness to learn.

"If he continues on his trajectory, I think he'll be a really, really good player. I have been super proud of him -- he's been a huge addition for us."

49ers' deep rookie class keeps getting better, as Connor Colby steps into starting role