SANTA CLARA, Calif. -- An MRI exam revealed that four-time San Francisco Pro Bowl linebacker Patrick Willis has a Grade II hamstring strain and "could miss a little time," a league source told ESPN NFL Insider Adam Schefter.
Willis' status for Sunday's game at Arizona is unclear, but the source told Schefter that the injury will heal on its own.
Coach Jim Harbaugh said he didn't know the severity of Willis' injury or whether it could sideline him for NFC West champion San Francisco (10-2), which clinched its first division title and playoff berth since 2002 with a 26-0 win over the St. Louis Rams on Sunday.
"Nothing to share," Harbaugh said. "I'm not in Patrick's body. It's hard to say with any injury, including a hamstring. ... I guess I was more relieved that it wasn't a knee. That area that they were working on -- I was concerned about that, for Patrick and for our team."
Willis landed awkwardly after missing a tackle on fullback Brit Miller during an 18-yard completion from A.J. Feeley with 4:28 left in the first quarter. Willis was briefly down before being helped off the field by trainers. He watched the rest of the game in a sweat suit.
Willis, the heart of the team's top-ranked run defense, has 93 tackles, two sacks, a career-best four forced fumbles and an interception. In May 2010, he received a $50 million, five-year contract extension that takes him through the 2016 season and includes $29 million in guaranteed money. He earns $10 million per season.
"I know Pat is a hard player and I was like, 'It has to be something serious for him not to just get back up and get back in the game,' " defensive lineman Ricky Jean Francois said.
The hard-hitting Larry Grant played well in Willis' place.
"He's got a high level of contact courage," Harbaugh said of Grant.
The 49ers can afford to rest Willis to ensure he is full strength for the playoffs next month, but Harbaugh insists the fact they already won the West would have no bearing on playing time for guys like Willis or running back Frank Gore.
"The trainers and Patrick will give feedback there and make a decision," Harbaugh said.
In the final week last season, Willis underwent a second surgical procedure for a broken right hand and missed a start and game for the first time in his career. That after he played two games with a bulky cast on his hurt hand.
Voted to his fourth Pro Bowl in as many seasons in 2010, Willis sat out football's all-star game injured for the second year in a row.
Gore is confident Willis will return in short order.
"I was kind of nervous, kind of scared because we need Pat," Gore said. "He's a big part of this team, he's our leader. I feel real good with 52 on the field, you know? I'm happy it was a hamstring and he'll be fine."
Information from The Associated Press contributed to this report.