SANTA CLARA, Calif. -- San Francisco 49ers quarterback Jimmy Garoppolo's latest high right ankle sprain wasn't so much an aggravation of his previous injury as a whole new one, which is why he's expected to miss about six weeks and could be out for the rest of the season.

49ers coach Kyle Shanahan said Tuesday that the ankle sprain Garoppolo suffered in Sunday's loss against the Seattle Seahawks was "essentially a whole new one, just on the same foot," and this sprain is considered more serious than the one he suffered in Week 2 against the New York Jets.

Because of that, Garoppolo is in the process of gathering multiple medical opinions before deciding whether he will need surgery, which would end his 2020 season.

"When you have those high ankle sprains, you reinjure them a lot," Shanahan said. "But it was different the way that he did it, so it's basically doing it all over again in a different way. ... Whatever's best for him, long term, is obviously what we'll decide on. But if he does need the surgery, then it'll be all year; if not, then hoping six weeks, get a chance to get back at the end."

Asked whether he felt like Garoppolo was brought back from the first ankle sprain too soon, Shanahan indicated that wasn't really the issue, and though Garoppolo struggled mightily in his return against the Miami Dolphins, he'd progressed in the two weeks before the loss to Seattle.

"I don't regret bringing him back any of the times," Shanahan said. "Obviously, the way the Miami game went, if I could've seen that before the game, we could've avoided bringing him back, but I thought he gave us a real good chance to win and we didn't think he was risking injury by going further with it and he didn't. That's why he was able to come back the next two weeks even healthier. But he redid something in the Seattle game and it was bad ... it ended up being worse than we thought."

At some point soon, Garoppolo will go on injured reserve, which means he has to miss at least the next three games, including Thursday night's game against the Green Bay Packers. The same is true of tight end George Kittle, who suffered a fracture in his foot that initially didn't show up on X-rays on Sunday but did upon further testing on Monday.

Kittle, who missed two games with a sprained knee earlier this season, is known for returning sooner than normal injury timetables, but Shanahan indicated that probably won't be the case this time.

"I think they told me eight weeks," Shanahan said. "Kittle says two but that's how he rolls. But that's why he'll go on IR. If it's better than what they're saying then he'll have a chance to come back this year, but eight weeks is eight weeks."

Shanahan also ruled out receiver Deebo Samuel (hamstring), running back Tevin Coleman (knee sprain) and linebacker Demetrius Flannigan-Fowles (hamstring) for Thursday night against Green Bay.

Backup Nick Mullens will start for Garoppolo at quarterback against the Packers, with Ross Dwelley likely to step in for Kittle. Tight end Jordan Reed, who is working back from a knee sprain, could also be in the mix, though his participation in Tuesday's practice was expected to be limited.

Shanahan also indicated that practice squad running back Austin Walter could be promoted this week to join JaMycal Hasty and Jerick McKinnon as the only healthy running backs available.

The Niners' relentless spate of injuries have taken their toll, but the team has little time to dwell on them with another game in two days.

"We had an idea on the injuries but we kept getting worse news on that stuff, which was tough," Shanahan said. "But eventually you run out of hours in the day, you have got to get to game-planning... All we can think about is one thing right now."

Additionally, the 49ers waived wide receiver Dante Pettis and defensive lineman Jullian Taylor. Pettis was a second-round pick in 2018. Taylor had spent the season on the physically unable to perform list after tearing his ACL in December of last year.