The most important development of Stanford's season took place long before the Cardinal upset Oregon or smothered UCLA -- long before the first kickoff or the opening of training camp.
It came out of nowhere on Feb. 11, packing a wallop: Running back Tyler Gaffney was returning to Stanford after a year playing minor league baseball.
"He was the perfect fit," Oregon State coach Mike Riley said recently. "Talk about the right place, right time and right decision for Stanford football."
Nine months later, Gaffney is a one-man wrecking crew for the 10th-ranked Cardinal, which is short on playmakers at other positions.
The senior from San Diego has scored 80 percent of Stanford's touchdowns and accounted for 45 percent of its yards from scrimmage in the past four games.
"He's allowed them to say, 'Here's who we are,' " said Todd Husak, the Stanford radio analyst and former all-conference quarterback.
"He earns the tough yards. When you have a guy like that, it changes your strategy for attacking defenses."
Gaffney ran for a career-high 171 yards in a mid-October victory over No. 9 UCLA, followed with a three-touchdown performance in the win at Oregon State, then set a school record with 45 carries in the upset of No. 2 Oregon.
His dazzling performance last week at USC included a see-it-to-believe-it run in which he was stopped cold behind the line of scrimmage, broke free of a swarm of defenders and charged 35 yards for a touchdown.
But the best gauge of Gaffney's value isn't the statistics or highlights. It's imagining Stanford's season without him.
"The Oregon game would have looked a lot different," Husak said.
"Going into the year, the big question was how they were going to replace Stepfan Taylor as an every-down back. They lost (tight ends) Zach Ertz and Levine Toilolo, and they needed to find some offense.
"Gaffney had been off for a year. He was looked at as being able to add to depth. Nobody knew he'd be this outstanding."
Gaffney, who grew up in San Diego, spent three seasons as a backup to Taylor and Toby Gerhart and three springs as an outfielder on the Cardinal baseball team.
After the Pittsburgh Pirates selected Gaffney in the 24th round of the 2012 draft, he left school and reported to the organization's Single-A affiliate, the State College Spikes.
He hit .297 and waited for word of a promotion.
"I wanted to move up in the system," he said. "They never said anything, and I didn't want to give them an ultimatum."
Gaffney never lost contact with Stanford and broached the subject of returning with coach David Shaw. It became official in February, but Gaffney couldn't re-enroll until the spring academic quarter -- just in time for the second session of spring football practice.
"You could see when he came back that he was motivated like never before," quarterback Kevin Hogan said. "I think he wanted to prove something to himself and everyone else."
Gaffney split carries with Anthony Wilkerson in September. But when Stanford entered the heart of conference play, Gaffney became the every-down back because of his reliability in pass protection -- "I understand what the offensive line is trying to accomplish and where I fit in," he said -- and his knack for turning nothing into something.
Largely because of Gaffney, who has lost yards on just eight of his 235 carries this season, the Cardinal leads the country in the fewest number of negative plays (3.2 per game).
"The thing with Tyler is that he breaks tackles if plays aren't blocked properly," Shaw said. "He takes a lot of pride in getting back to the line of scrimmage."
It's impossible to project Stanford's success in a Gaffney-less existence. But it seems reasonable to assume the Cardinal would have been forced to rely more heavily on Hogan, who ranks seventh in the conference in passing efficiency, and a group of inconsistent receivers.
"It's the chicken-and-egg thing," Husak said. "Are they not asking Hogan to do more because they're more comfortable with Gaffney? Or is Gaffney getting more carries because they aren't comfortable with the passing game?"
Clearly, a program built on the power running game would have been forced to tweak its approach had Gaffney not returned.
Wilkerson doesn't have Gaffney's brute strength or durability; he's more suited for toss sweeps than up-the-gut runs.
The other tailbacks (Ricky Seale, Remound Wright and Barry Sanders) are smaller and wholly unproven when it comes to gaining the toughest yards -- the yards that Gerhart and Taylor gained by the thousands during Stanford's ascent to the national stage.
"They have such a good picture on their identity for what they want to do," Riley said. "But you have that great piece. Great running backs make the other stuff come alive."
116th big game
Cal (1-10, 0-8 Pac-12) at Stanford (8-2, 6-2),
1 p.m. Saturday, Fox Sports 1
Stanford tailback Tyler Gaffney is averaging 157.8 yards per game and has eight touchdowns in the past four games. Here's the breakdown:
Opponent Result Stats
UCLA W, 24-10 171 yards, 2 TDs
at Oregon State W, 20-12 145 yards, 3 TDs
Oregon W, 26-20 157 yards, 1 TD
at USC L, 20-17 158 yards, 2 TDs