The difference between 4-12 or an 8-8 record for the San Francisco 49ers will come down to the play of their cornerbacks.

You read that right. Cornerback, not quarterback.

A year after they finished 31st in total offense, my money is on the 49ers taking a seismic leap to the middle of the pack. Everywhere he’s been, Kyle Shanahan has been able to move the football up and down the field in a variety of fashions. Brian Hoyer knows this offense well, is a smart decision-maker and will surprise people with his deep ball accuracy. The offensive line is the strength of the team. Pierre Garcon adds consistency. The offense is nowhere near a finished product, but it won’t be problematic with Shanahan pulling the strings.

Defense, on the other hand, is an entirely different story. Coordinator Robert Saleh will be calling plays for the first time. Other players on the roster are being asked to try new positions — Jimmie Ward is at single-high safety, Arik Armstead is now a pass rushing LEO. Expecting a seamless transition for Ward and Armstead is being naïve. John Lynch drafted Solomon Thomas and Reuben Foster to stabilize a front-seven that gave up a franchise record in rushing yards allowed. A healthy NaVorro Bowman should change that in the middle.

It’s becoming clear which position group Shanahan and Lynch will have anxiety about heading into September. Cornerback. There’s young talent at the position, but also enough inexperience where issues could arise on a weekly basis.

If the season started today, Rashard Robinson would be penciled in as the No. 1 cornerback on the team. Third-round pick Ahkello Witherspoon will compete with Dontae Johnson for the second spot in August. K’Waun Williams, Will Redmond, Keith Reaser and Prince Charles Iworah will vie to play nickel back. Rookies Adrian Colbert, Lorenzo Jerome and Zach Franklin are on the 90-man roster, but don’t expect much.

Considering he was a fourth-round pick playing for the first time, the 6-foot-2 Robinson was mostly excellent in 2016. However, there is no true way to prepare for the No. 1 position the 49ers are putting him in. Is he really ready to slow down Dez Braynt, T.Y. Hilton, Odell Beckham Jr., DeAndre Hopkins, Doug Baldwin and Larry Fitzgerald week after week? Practice reps against Garcon will help Robinson, but the only true way for a NFL cornerback to grow up is on Sundays. The 49ers are putting an enormous amount on Robinson’s plate.

The real conundrum for Shanahan and Saleh will come when making the No. 2 cornerback decision. The lengthy 6-foot-3 Witherspoon has more potential than Johnson, but throwing him out on an island come September could destroy his confidence. The Colorado product likely gives the 49ers a better chance to win Week 1 against the Panthers, but slowly developing him could pay dividends down the road. It’ll be a tricky choice.

Johnson started three games each in 2014 and 2015, but fell out of favor with the last coaching staff. He logged just 100 snaps on defense a year ago, equivalent to just 8.6 percent. To compare, he played 371 snaps on special teams. Johnson hasn’t exactly been in an incubator waiting for this starting opportunity. There would be clear struggles from the fourth-year pro, too.

Remember this about San Francisco’s new defensive scheme: Eric Reid is going to be playing mostly as a box safety. With only Ward playing deep, the cornerbacks have less help in this system than most others around the league. Reid hasn’t gotten a lot of credit over the years for putting out fires in the back end of the secondary. He’s an instinctive communicator on the field who can cover up mistakes from his teammates. Now, he’ll be more focused on levying hard hits, patrolling the middle and stopping the run rather than providing insurance for his fellow cornerbacks.

This is where drafting Solomon Thomas at No. 3 overall has an impact on the rest of the team. If he’s a difference-maker right away — penetrating the middle, hammering running backs and forcing 3rd-and-longs — San Francisco’s cornerbacks will be the direct beneficiary. If Thomas struggles to make waves in his rookie season on the defensive line, the secondary will feel the brunt of his missteps. Drafting a safety at the top of the draft would’ve left Ward at cornerback alongside Robinson. That’s clearly a more formidable duo on the outside.

Anxiety could turn to elation if the talented Robinson makes a big leap in Year 2. If he’s consistently slowing down No. 1 wide receivers or at least keeping them out of the end zone, the 49ers will have a chance at 8-8. The best case scenario for Witherspoon is to have a rookie season just like Robinson did — keeping his side of the field mostly quiet without getting burnt. Easier said than done.

Shanahan and Lynch are aware of the reality: It’s impossible to restock an entire roster in the offseason. Developing young players like Robinson and Witherspoon often lasts a few years, and you have to be willing to take the lumps the come with it. A year after opponents rammed the football down the 49ers’ throats, I’d expect them to take the opposite strategy to begin the season. Offensive coordinators will strategize to expose San Francisco’s young cornerback duo. Robinson, Witherspoon and Johnson have to prove they can cover. It’ll be plain and simple.

Cutting Tramaine Brock on suspicion of domestic violence was the right decision. Shanahan wanted to make it clear the 49ers weren’t sending a message, they will just always try to do the right thing. Even though Brock never played at a Pro Bowl caliber level, there will become a time this season where he’s missed. An injury to Robinson, Witherspoon or Johnson would be borderline devastating. That’s not a position Shanahan or Lynch want to be in, but again, this is Year 1 of a massive rebuild.

You can never have enough good cornerbacks on the roster and the 49ers might find out the hard way in 2017. Or Robinson and Witherspoon’s development could lead to a few extra wins.