SANTA CLARA, Calif. -- Through a barrage of seemingly endless questions about his quarterback situation, San Francisco 49ers coach Kyle Shanahan has remained adamant that the plan was to ride with incumbent Jimmy Garoppolo while rookie Trey Lance develops.

But if the right calf injury Garoppolo suffered in Sunday's 28-21 loss to the Seattle Seahawks causes him to miss games, Shanahan will have no choice.

"We've been playing with rookie running backs since the second play of the first game and we've got a rookie quarterback right now," Shanahan said. "Hopefully, Jimmy is better, and not as bad as we think, but Trey is here for a reason. You want to give him time, you want him to be fully ready, but you don't always have that luxury."

Garoppolo said he was set to have an MRI on his right calf Monday morning. The Niners are also concerned about left tackle Trent Williams' right shoulder and kicker Robbie Gould's right groin, both of which will require further testing.

Williams departed the game in the fourth quarter on a cart and did not return. Gould's groin issue happened during final warmups and he missed the game as punter Mitch Wishnowsky handled kicking and punting duties.

Garoppolo said he first felt the injury on the game's opening series. He didn't know who it was, but Garoppolo said it felt like someone stepped on his right calf. As the first half wore on, Garoppolo said the calf continued to tighten and he could feel the pain working its way down to his Achilles.

While Garoppolo doesn't think it's anything as serious as an Achilles tear, he did indicate it could cost him some time.

"It feels like just the calf right now," Garoppolo said. "I'm hopeful it will just be a couple of weeks or something like that."

Even if it is just a couple of weeks, that would mean Lance would make his first regular-season NFL start next week against the undefeated Arizona Cardinals on the road. Which means the Niners would have to make some changes to the game plan to cater more to Lance's skill set and limited experience in the full offense.

It would also mean the Niners would have time to plan for Lance as the starter, something they didn't have on Sunday. Garoppolo continued to play through the first half on the injured right calf, and it wasn't until a minute or two before the third quarter began that he was ruled out.

That decision hit Garoppolo hard, especially considering his lengthy injury history and the presence of the highly drafted Lance.

"It's just tough," Garoppolo said. "I've been in this situation too many times and it's getting real old."

Garoppolo finished his day 14-of-23 for 165 yards with a touchdown but started 6-of-6 for 70 yards and a touchdown on the first drive. But Garoppolo said he could feel the calf affecting his throws as the half went on. He went 8-of-17 for 95 yards and an interception after the initial drive.

Lance's first steady work with the starters got off to a rough start with a pair of three-and-outs and a couple of ugly incompletions. But he hit receiver Deebo Samuel for a 76-yard touchdown on his third series on a pass in which Samuel had 17.6 yards of separation at the time of the catch, the most for a receiver on a touchdown this season, according to NFL Next Gen Stats.

Lance seemed to settle in a bit from there. Trailing by two scores in the fourth quarter, Lance directed another scoring drive, hitting Samuel for another score, this time from 8 yards out. He finished 9-of-18 for 157 yards with the two scores for a passer rating of 117.1. He also rushed for 41 yards on seven attempts.

He said he felt like his first extended time came with some good and some bad.

"Missed some big ones, missed some easy ones, missed some checkdowns, just definitely a few plays that I want back," Lance said. "I'm going to go back and watch it tonight and I think I'll learn a lot from that."

Shanahan offered a similar assessment of Lance's performance with an eye toward the type of improvement that will be necessary if Garoppolo doesn't play next week and Lance is indeed needed for his first start.

"He got thrown in not expecting to get in," Shanahan said. "I thought he ran the ball well, hit some passes, obviously missed some passes, was in a tough situation there when we were down two scores. He did a good job of moving the chains a couple times with his legs. But it looked like a typical first game."