SANTA CLARA, Calif. -- Minutes after he was diagnosed with a broken left fibula that will keep him out for six to eight weeks, San Francisco 49ers offensive tackle Joe Staley stood on crutches in Cincinnati's visiting locker room on Sunday, vowing to do whatever he can to help his replacement play at a high level.

It didn't take long for Staley to back up his words. On Tuesday night, Staley invited rookie tackle Justin Skule to his house to begin a crash course on playing left tackle in the NFL.

"He's been awesome since I've been here, just taking me under his wing and giving me a little tricks of the trade," Skule said. "And he's done a phenomenal job just these past couple days, giving me pointers, showing me about how he goes through his process and everything like that. I couldn't ask for a better mentor."

This week, Skule will take all the help he can get as he prepares for his first NFL start. And Skule isn't just stepping into the starting lineup -- he's filling the left tackle spot that Staley has held down for the better part of the past 13 years, making six Pro Bowls.

Staley is so ingrained in the 49ers' fabric that he was signed to a two-year contract extension through the 2021 season that should allow him to retire with the team. Aside from maybe quarterback Jimmy Garoppolo, there's no more valuable player on the roster.

This means it's not as much about replacing Staley as trying to keep Garoppolo upright while Staley recovers. That the Niners intend to give Skule the first opportunity serves as a vote of confidence in the sixth-round draft pick out of Vanderbilt.

The rumor mill accelerated into overdrive soon after Staley's injury, with speculation about the Niners pursuing Washington's Trent Williams via trade or signing a veteran free agent such as Sam Young (which they did on Friday). Skule will start, though, with versatile Daniel Brunskill the presumed backup.

San Francisco coach Kyle Shanahan said earlier this week that while the Niners would scan available options, he and general manager John Lynch didn't see attainable tackles who were an obvious upgrade over Skule.

"We would love to if there were options," Shanahan said. "You look at anything that's possible. I don’t think there's a ton of great options out there that are possible."

Instead, the 49ers will give Skule, who was the team's fourth tackle before a season-ending ankle injury to swing tackle Shon Coleman, an opportunity to start against the Pittsburgh Steelers on Sunday as they attempt to get to 3-0 for the first time since 1998.

Skule didn't come with much fanfare, but the Niners liked him because he was a solid, productive player in college. Skule started the final 40 games at Vanderbilt after missing five games to injury in his freshman year. He held his own against some of the nation's elite defensive linemen and earned a reputation for outsmarting opponents as a three-time All-SEC Academic Honor Roll selection.

While Skule lacks the athleticism and pedigree of fellow tackles Staley and Mike McGlinchey, he has impressed teammates and coaches with his quiet, calm approach.

"He's been very consistent," Garoppolo said. "Smart guy, came into the huddle during the game, didn't blink or anything. Just one of those guys, he does the right thing and he's in the right spot. He's been the same guy every day, and you've got to appreciate that."

Last week, Skule played the final 19 snaps. Because the Niners had a huge lead, only one of those was a pass-blocking opportunity, and even that was a quick toss to receiver Deebo Samuel.

This week, Skule can count on being tested early and often by a Steelers defense that is seventh in pass-rush win rate at 55.6%. Bud Dupree likely will get the majority of the opportunities against Skule, who steps in for an offensive line that is seventh in pass-blocking win rate (64.7%) and has yielded just one sack and three quarterback hits.

Suffice to say, the Niners will have to provide Skule with help from backs and tight ends as well as asking Garoppolo to get the ball out quick, though there will be times when Skule has to hold his own.

"You still have to account for that stuff," Shanahan said. "There are a lot of other people on the field that you've got to take care of and try to help too. I'm confident that Skule will come in and by no means will he be perfect, just like Joe isn't, but the game's not too big for him. I know he'll battle and give us a chance to win."

Skule seemed to take his new place in stride Wednesday afternoon. He acknowledged that there will be nerves, though those will go away after a few snaps. It's a lesson right guard Mike Person reminded him of last week, when Person told Skule to take a deep breath when he entered for Staley.

With the bye coming after Sunday, the 49ers could use the Pittsburgh game to evaluate Skule. If things don't go well, perhaps they'll go with Young or look for other options. For now, though, they are giving Skule a chance to make the job his until Staley returns.

"It's a phenomenal opportunity," Skule said. "And I look to take advantage of it. It's very meaningful, and so I hope to make them right and go out and play the best football I can."