The Cardinal are the only Pac-12 team that Mariota has never defeated heading into Saturday's meeting at No. 5 Oregon.
This matchup will likely decide the conference's North Division, as Oregon (7-1, 4-1) leads Stanford (5-3, 3-2) by one game.
The Cardinal have dropped three regular-season games for the first time in five years, though Helfrich doesn't see this as an easy assignment.
"They've looked pretty good all year to me," he said. "They've had some tough games against some very, very quality opponents."
Oregon is wary after seeing bids for unbeaten seasons end each of the last two years against Stanford. The Ducks were 10-0 before a 17-14 overtime home loss in 2012 and 8-0 last year before suffering a 26-20 road defeat.
Mariota is a contender for the Heisman Trophy, leading the nation with 10.47 yards per pass attempt. He's tied for third in the FBS with 24 touchdown passes and is ninth in completion percentage at 68.8.
His problem is that he's been fairly average against Stanford, completing 57.7 percent of his passes while getting sacked six times in two defeats. That doesn't mean that Shaw believes he has a magic formula to slow down Mariota, Oregon's all-time leading passer with 8,625 yards.
"At times, we've been able to contain him, we've been able to harass him," Shaw said. "But in every game, there's a streak where you can't do anything about it. He gets out of the pocket and takes off. He makes a couple of great throws. He moves the team down the field in three plays and scores a touchdown."
The Cardinal coach remembers the explosiveness displayed by Mariota and the Ducks last year. Stanford led 26-0 in the fourth quarter before Oregon made it close.
Stanford kept Mariota off the field by holding the ball for 42 minutes last year and 37 minutes in 2012, rushing for a total of 474 yards.
Kevin Hogan has provided steady play at quarterback, throwing for 211 yards two seasons ago and helping the Cardinal convert 14 of 21 third downs last year. Hogan turned in a rushing touchdown in both games.
"Kevin's played probably two of the best games of his career against us the last two years," Helfrich said. "He's a really competitive guy, he's been extremely accurate, kind of along the lines of (UCLA quarterback Brett) Hundley in terms of how he runs, he's hard to bring down. He's played really well."
Shaw did not reveal whether he would stick with the spread-style offensive approach he used in last Saturday's 38-14 win over Oregon State. He employed fly sweeps, read-options and multiple wide receiver sets rather than rely primarily on a rushing attack that ranks fourth-worst in the conference at 141.3 yards per game.
''We just have to do whatever our guys can do,'' Shaw said. ''If we can get like we did last year and get to our big personnel to be able to run the ball efficiently, hey, that's great, we can do that. If we have to spread it out and run and throw and move the football, hey, we have to do that."
Freeman has been a catalyst during the win streak with 402 rushing yards and eight touchdowns. He leads the Pac-12 with 13 TDs on the ground.
The freshman has developed into the primary running back over Thomas Tyner, who sat out last week with a shoulder injury. Tyner's status for Saturday is unclear.
Stanford features a special teams ace in Ty Montgomery, who scored on a 50-yard punt return last weekend. It marked the second time in 2014 that he ran back a punt for a score and the fifth return TD of his career.