Stanford feared the worst, and the news was confirmed today that junior middle linebacker and leading tackler Shayne Skov will miss the remainder of the season with a knee injury.
Skov left the game in the second quarter of Saturday's 37-10 win over Arizona in Tucson after his leg was caught under him during a tackle.
"It's a big loss for our football team, but we have great leadership on defense from players such as Michael Thomas, Delano Howell and Chase Thomas," said coach David Shaw, who didn't disclose details of the injury. "We've also recruited very well recently and now it is time for guys to step up."
Stanford, which moved up to No. 5 in the AP poll this week has a bye this weekend. Stanford's next game will be Oct. 1 against UCLA.
Skov was assisted off the field by trainers and then carted from the sidelines before the end of the first half.
Skov had one tackle for a loss before he left the game. His 19 tackles lead Stanford this year. He was also the team's leading tackler last season.
Following the game, one player referred to Skov as "the leader" of the defense. Stanford's run defense ranks second nationally through three weeks, allowing 36 yards per game.
Skov, from Guadalajara, Mexico, was a Pac-10 honorable mention performer last year after recording a team high 84 stops -- including 50 solo tackles. He shined in Stanford's victory in the Discover Orange Bowl over Virginia Tech last season, notching 12 tackles and three sacks.
The responsibility likely falls to redshirt sophomore Jarek Lancaster and true freshman James Vaughters to fill in for Skov.
Outside of Heisman Trophy hopeful Andrew Luck, there might not have been a more important Cardinal player than Skov, the unquestioned leader of the defense and a projected NFL draft pick.
"He's the emotional leader of the defense. You can't replace that," safety Michael Thomas, the defensive captain, said after the game in Arizona.
Skov's contributions go well beyond tackles and sacks.
The middle linebacker relays most of the calls to teammates and oversees audibles at the line of scrimmage. The signature eye black that Skov spreads liberally around his face and his mini-mohawk cut also had become staples on the Stanford defense, giving an academics-first university a bit of an edge.
The injury could have significance beyond this season.
The 6-foot-3, 251-pound junior was expected to enter the NFL draft in April, although now he could delay the jump to return to Stanford next year. His brother, Patrick, is also a freshman fullback on the team.
Kevin Gemmell covers Stanford football for ESPN.com. Information from The Associated Press was used in this report.