With starters returning, 49ers believe 'ceiling is however high you want to make it'

Yates: Alexander injury will test 49ers depth (0:57)

Field Yates considers Kwon Alexander's injury a huge blow to the 49ers and says it will test their depth because they are a top heavy team. (0:57)

SANTA CLARA, Calif. -- On Friday, the San Francisco 49ers got terrible injury news when an MRI revealed that linebacker Kwon Alexander will miss the rest of the season because of a torn pectoral. But after a weekend of rest, the injury news is about to get a lot better.

As the Niners prepare for a Monday night showdown against the Seattle Seahawks (8:15 p.m. ET, ESPN), they're expected to begin welcoming back a quartet of starters from previous injuries. It's a group that includes tackles Joe Staley and Mike McGlinchey, fullback Kyle Juszczyk and cornerback Ahkello Witherspoon.

From that group, Staley and Juszczyk look most likely to return, but it's possible McGlinchey and Witherspoon will also be back.

While the loss of Alexander is big, the return of so many pieces has the 49ers believing they can improve on their 8-0 start.

"The ceiling is however high you want to make it," defensive tackle DeForest Buckner said. "We've been playing really well with guys down and the guys have been stepping up really well. To have some of those guys back is going to be very exciting. We've been missing them for a couple weeks and the fact we're 8-0 without them just shows the depth on this team."

Starting with Staley's broken fibula in Week 2 against Cincinnati, the Niners had a streak of three games where they lost at least one starter to injury. Witherspoon injured his foot the following week against Pittsburgh, and McGlinchey and Juszczyk followed with knee injuries in the next game against Cleveland.

Each time, there were questions about the Niners' ability to replace each player. What's happened since has been one of the most important developments of this season. With unproven youngsters such as tackles Justin Skule and Daniel Brunskill, cornerback Emmanuel Moseley and tight end-turned-fullback Ross Dwelley, the Niners proved deeper than they've been in recent seasons.

All four replacements have played better than expected, with the help of creative scheming by the coaches to help make the transitions more manageable.

"Just being able to have so many guys that can fill in, it's incredible," tight end George Kittle said. "Being able to fill the cog in the machine and just keep rolling. Just being able to get those reps, that was huge. That's how you get better at the game is get live reps against guys."

Getting the likes of six-time Pro Bowler Staley, three-time Pro Bowler Juszczyk and developing building block McGlinchey back should allow coach Kyle Shanahan to open his playbook further. The Niners had been heavy on '21' personnel, with Juszczyk playing all over the formation, but they've cut down on it since his injury. Dwelley has proved a solid replacement, but his playing time at fullback hasn't been the same as what the 49ers do with Juszczyk.

While the two tackles and Juszczyk have healed, the 49ers have added more looks out of '11' personnel (three receivers, one tight end and one running back) and '12' personnel (two receivers, two tight ends, one running back). They've even turned to some '13' personnel with three tight ends.

That experience mixing things up should serve the Niners well moving forward and allow Shanahan to get even more creative.

"We've found two guys who can do it and you wouldn't have known that if Joe and Mike were playing the entire time," cornerback Richard Sherman said of the tackles. "You'd still have a lot of questions like, 'Oh, if one of these guys gets hurt, who is going to step in?' Those questions have been answered, and even with Juice coming back it gives us even more versatility because the guys who have had to fill in for Juice are going to be able to be just as versatile. Kyle [Shanahan] is going to be able to call the offense as he has whether Juice is in or not."

As for the defense, Moseley has been so good in Witherspoon's stead that some have wondered if Witherspoon will have to win his job back. Shanahan and coordinator Robert Saleh have said they don't believe in players losing their job to injury, but they trust Moseley.

Suddenly, a cornerback group that seemed to lack depth behind Sherman has multiple options.

"Emmanuel has been fantastic and been one of the best corners in the league statistically," Sherman said. "Anytime you are playing like that, it shows depth ... it gives everybody confidence in what we can do with who we have."

The Niners don't play on the road again until Dec. 1. The schedule will get tougher, but the Niners believe that as it does, they will only get better as they get healthier -- a scary proposition for a team that hasn't lost yet.

"We'll heal up, get some guys back and make a run at this thing," quarterback Jimmy Garoppolo said. "We've got a long way to go but we're on the right track."