In an offseason marked by the Vikings' yeoman effort to keep their defensive core together despite salary cap constraints, defensive tackle Sheldon Richardson nonetheless stood out as perhaps the most significant one that got away.

Richardson signed a three-year, $36 million contract with the Browns in March, again creating an opening at three-technique tackle after posting 4½ sacks last season. The Vikings brought back Shamar Stephen on a three-year deal to help replace Richardson, but so far in training camp they haven't been able to see much of their reworked defensive interior.

The Vikings' defensive line depth has been tested early in camp, with Stephen still on the physically-unable-to-perform list and Linval Joseph limited because of offseason surgery. Coach Mike Zimmer has declined to elaborate on Joseph's injury, but the nose tackle appeared to be doing rehab work on a shoulder in practice Monday.

It's left Hercules Mata'afa — having returned from a torn ACL last year — at three-technique tackle, with third-year man Jaleel Johnson at nose tackle in the Vikings' first-team defense during the first padded practices of camp. The Vikings got second-year tackle Jalyn Holmes back from a leg injury on Monday and have been finding work for rookie tackle Armon Watts as well.

The young tackles struggled against the Vikings' interior offensive line at times on Sunday, particularly at the end of practice, when Dalvin Cook breezed into the end zone behind a Josh Kline block in a red zone drill.

"Hercules [Sunday], he was up and down," Zimmer said. "Had some good plays and some bad plays. Watts [was] pretty much the same."

While Joseph's injury isn't expected to be a cause for great concern, and Zimmer said last week he didn't think Stephen would be out long, the Vikings nonetheless have to make do without their starting defensive tackles for now. Their younger players, in the meantime, can use it as an opportunity.

Tom Baker for Star Tribune
Video (04:12) Reporters Ben Goessling and Andrew Krammer say that the newly reformed offensive line isn't being challenged enough at training camp because key defensive players are injured.

After playing all 16 games for the first time last season, Johnson spent his offseason in the Twin Cities so he could continue working out at the Vikings' facility and stay in touch with the team's training staff. "As 'Uye' [strength and conditioning coach Mark Uyeyama] talks about, it's staying close to home," he said. "Whatever we did in the spring, and the things that we do as a defensive line unit, just keep doing it every single day. I've got the strength staff here, and I've got the equipment in the weight room, so what more could I ask for?"

The timing of Mata'afa's injury — he tore his ACL during organized team activities before the 2018 season — gave him time to recover and be fully healthy before the Vikings' 2019 training camp. The key to his progress, Mata'afa said, was not making his injury an issue that lingered in the back of his mind all offseason.

Zimmer spoke highly of Mata'afa's progress and his build this offseason. "His body lean is extremely low. He's got great post stuff," the coach said in June. "Let's not put him in Canton yet, but he's doing a nice job with the things we're asking him to do."

Though the Vikings' young tackles will likely be relegated to rotational roles once Joseph and Stephen return, injuries could force the team to delve into its depth at any time. Given the number of unproven players the Vikings have behind their starters, the early days of camp could provide some valuable experience — even if the immediate results are a little uneven.

"I was definitely in that position once," linebacker Anthony Barr said. "I understand it can be a little nerve-racking — they have a lot of things to process. But the best thing they can do is just communicate. Even if they're wrong, just ask questions, and don't be afraid to mess up. It's going to happen; just try not to repeat it."