SANTA CLARA, Calif. -- The San Francisco 49ers' rookie minicamp starts on Friday, but before it does, the team took care of some off-field business.

The Niners welcomed their rookies to town on Thursday afternoon, and upon arrival and physicals they finalized contracts with 18 undrafted free agents, signed six drafted rookies to their first NFL contracts and welcomed a handful of tryout players to the fold. But the on-field work is now set to begin.

With that in mind, here's a look at a few things worth watching in the on-field work:

1. A Solomon Thomas cameo: Because of NFL rules, the No. 3 overall pick will get a taste of the Niners playbook and practice methods this weekend but then will have to disappear until his class at Stanford completes its school year. It's a frustrating rule for the player and the team but it's not unfamiliar, either. Last year, the Niners had to wait on guard Joshua Garnett to do the same thing before he could come to work. Thomas remembered seeing Garnett around campus as he worked out and tried to stay sharp for when he finally arrived.

Nobody is asking Thomas to learn everything in one weekend, and if he's not fully up to speed when the Niners begin training camp, that's OK. But San Francisco is likely to look to him to start from the beginning of the season, something Garnett was unable to do because he was behind the curve.

So for Thomas the next few days will be about absorbing what he can and doing what he can to incorporate those things into his workouts when he's not at Levi's Stadium. Thomas plans to lean on Garnett to handle his time away.

"I will be talking with Joshua and other guys on the team," Thomas said. "Talking to the coaches as much as I can to learn as much as I can while I’m away, while I can’t be at the facility. So, I’m going to do whatever I can to just continue to learn and get better while I’m away but still try my best to be here in some way.”

2. Ahkello Witherspoon's effort to find physicality: The 49ers clearly have a need for another starting outside cornerback after releasing Tramaine Brock. Dontae Johnson has been working there, but third-round pick Witherspoon should have a chance to earn early playing time. The biggest key to him winning that opportunity? A more physical approach.

Witherspoon had a whopping 22 pass breakups last year at Colorado and coverage skills aren't the reason he fell to the third round. But he struggled in run support and tackling in general was an issue. It's something even general manager John Lynch pointed out after drafting him.

"It needs to improve," Lynch said. "It’s something that Mike MacIntyre helped me a lot with because I like physical players and I think what helped is that I saw examples on film where he can do it and I think that’s why you go get guys like Solomon Thomas and Reuben Foster to draw that out of them. I always believe when you put people in a room that are exhibiting the way we want to play, the other guys have no choice if they want to be on the field. And so, I think Ahkello, it’s something that we raised to him and we didn’t hide from it. I said this and showed him the film: That bothers me, help me out here. And he was aware that it does need to approve and committed to making it improve. And in talking to Mike MacIntyre, he sees it as something that as long as it’s expected and demanded, he thinks really will [improve].”

The Niners won't be tackling live anytime soon, but Witherspoon acknowledged Thursday that he plans to be constantly working to become more physical so he can fit in with what they want to become.

3. Whither Reuben Foster?: Much has been made about Foster's surgically repaired shoulder, and coach Kyle Shanahan's comments on local radio Thursday seemed to rile up even more consternation though Shanahan was simply offering a worst-case scenario.

Speaking to KNBR radio, Shanahan said the worst-case scenario for Foster would be that the shoulder doesn't heal correctly and Foster would have to have another surgery that could potentially cost him his rookie season. The emphasis there is on worst-case as Shanahan pointed out that Niners doctors believe Foster's shoulder will hold up.

"It's not something that would hurt him long-term," Shanahan said. "So, the worst-case scenario: He wouldn't end up being able to play for us right away. But I still don't think how that would have changed how I personally felt about taking a guy with the 31st pick with the caliber of player that we took."

For his part, Foster has shot down reports that he needs a second surgery and said he's currently about 90 percent healthy. He is aiming for a return in training camp but he, too, will get a chance to get acclimated to the defensive scheme and start learning what his role will be over the next few days.