Technically, it’s not even fall yet. That starts Saturday when No. 7 Stanford visits No. 20 Oregon in a major Pac-12 North matchup.

The 5 p.m. game is big enough to warrant the top ESPN/ABC crew of Chris Fowler and Kirk Herbstreit, plus ESPN’s full “College GameDay” circus.

Yes, it’s early in the season, but Stanford’s defense has been surprisingly stout and leads the nation in fewest points allowed at 7.7 per game. The average would have been even lower if the subs hadn’t surrendered a touchdown pass to UC Davis on the game’s final play Saturday.

The Cardinal (3-0), however, won’t have one of their defensive stalwarts in Eugene. Head coach David Shaw said Tuesday that outside linebacker Casey Toohill is out this week. As usual, Shaw wouldn’t disclose the type of injury, but Toohill had his left arm immobilized as he watched practice Tuesday. Shaw said team doctors think he’ll be out four weeks.

As a result, redshirt sophomore Jordan Fox apparently will start, with sophomore Gabe Reid backing up both Fox and, on the other side, Joey Alfieri.

“It’s definitely going to be tough with Casey out,” Alfieri said. “He brings a lot to the defense.”

Toohill, a redshirt junior, had three tackles and two pass deflections in the 30-10 win over the Aggies. A new starter this season, he has taken part in 14 tackles, including a sack, and three quarterback hurries.

Along with linemen Michael Williams and Jovan Swann, Fox will make it three relatively inexperienced members of the front seven against the Ducks (3-0) and their offense, spearheaded by 6-foot-6 quarterback Justin Herbert.

The strength of the Stanford defense has been its linebacker group.

“I don’t know where it ranks in college football,” Shaw said, “but I’d be surprised if there are many people playing better than our linebackers are playing.”

Inside backers Sean Barton (who leads the team with 21 tackles) and Bobby Okereke have been aided by the much improved Mustafa Branch in a reserve role.

In previous years, Shaw said, the speedy Branch sometimes would find himself out of position. “With his age and experience, he’s right a lot,” he said. “He’s not guessing. He’s seeing it and reacting, and he’s attacking. He’s so quick and so explosive ... There’s no drop-off when he comes in the game.”

Although Toohill joins guard Foster Sarell, wide receiver Connor Wedington and outside linebacker Curtis Robinson on the injury list, tailback Bryce Love will be back after missing the Davis game. He apparently could have played Saturday, but Shaw wanted him to be fully ready for Oregon and the game at Notre Dame on Sept. 29.

“He’s 100 percent and feels great,” Shaw said.

Love will contend with a defense that has allowed just 2 yards per rush.

“They’re long, they’re athletic, they’re physical,” Shaw said. He pointed out that the Ducks have gotten off to big leads in all their games, making it tough for opponents to sustain a ground game.

“Even early in the games, when they’re not blowouts, it’s been difficult” to run on the Ducks, he said. “There’s no secret: We’re going to try to run the ball. It’s a good challenge for us this week.”

Briefly: Offensive tackle A.T. Hall, guard/tackle Devery Hamilton and backup quarterback Davis Mills are “day-to-day” with injuries, Shaw said, but he said all three probably will play Saturday. In the event Mills isn’t ready, Jack Richardson would become the No. 2 quarterback. ... UC Davis wide receiver Keelan Doss, who caught 13 passes for 106 yards, was targeted 25 times. Shaw said he hasn’t seen a player targeted that many times on the college level, although in the NFL, he coached against teams that did that for Hall of Famer Terrell Owens. ... Center Jesse Burkett, returning after missing the first two games, “steadied the ship,” Shaw said. “He did a really good job.”

Tom FitzGerald is a San Francisco Chronicle staff writer. Email: tfitzgerald@sfchronicle.com Twitter: @tomgfitzgerald