Stanford QB Keller Chryst to start vs. Oregon, but K.J. Costello will play

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Stanford’s quarterback rotation is over. Keller Chryst will start Saturday night against Oregon and play most of the game, head coach David Shaw said Tuesday.

In Saturday’s 23-20 win at Utah, Chryst and redshirt freshman K.J. Costello alternated, each getting about two series at a time. Costello also will play against the Ducks, but Shaw wouldn’t say how much.

He gave both quarterbacks a “B to B-minus” for the Utah game. All the Cardinal points came on Chryst-led drives, but Shaw said that wasn’t a factor in the decision. He just liked the redshirt junior’s overall play.

“Both guys made field-position-changing plays,” he said. “Both guys did a great job with their legs in the running game, Keller scoring a touchdown and K.J. getting two positive runs. Both made a couple of difficult throws.”

Chryst threw a third-down pass to tight end Kaden Smith for 54 yards and two to Trent Irwin for 25 and 19 yards. “Those are NFL throws,” Shaw said.

He said he anticipates that Chryst might be tempted to turn pro after this season “if he continues on the path that he’s on.” On the other hand, Shaw said, “he may want to come back and put more film out there, which is great.”

Said Shaw: “I think he can be one of the better quarterbacks in our conference. Hopefully, the second half of the season will bear that out.”

Stanford quarterback Keller Chryst completed 7 of 14 passes for 106 yards in the Cardinal’s win at Utah on Saturday night. Stanford quarterback Keller Chryst completed 7 of 14 passes for 106 yards in the Cardinal’s win at Utah on Saturday night. Photo: Rick Bowmer, Associated Press Photo: Rick Bowmer, Associated Press Image 1 of / 1 Caption Close Stanford QB Keller Chryst to start vs. Oregon, but K.J. Costello will play 1 / 1 Back to Gallery

Targeting costly penalties: No. 23 Stanford will be without its best defensive lineman, Harrison Phillips, and one of its top linebackers, Peter Kalambayi, for the first half of the Oregon game because of targeting penalties at Utah.

Redshirt freshmen Jovan Swann and Michael Williams figure to play more in Phillips’ spot. Sophomore Curtis Robinson and redshirt sophomore Casey Toohill probably will split up Kalambayi’s snaps, Shaw said.

He reiterated what he said after the game, that the targeting calls were correct because officials now are given wider latitude in making those calls to protect the safety of the players.

“We have to continue to emphasize taking the head out of those contacts, particularly on the quarterback and any defenseless player,” he said.

Injury updates: J.J. Arcega-Whiteside, one of the team’s top receivers, will be back Saturday night after missing the Utah game with an undisclosed injury.

Cornerback Terrence Alexander and linebacker Mustafa Branch remain out with injuries.

Taggart’s influence: Oregon’s first-year head coach, Willie Taggart, was Stanford’s running backs coach for three years under Jim Harbaugh and then Shaw. Taggart helped Toby Gerhart finish a close second in the Heisman Trophy voting in 2009.

He then left the Farm to become head coach at his alma mater, Western Kentucky, which had lost 26 games in a row. Three years later, WKU was bowl-eligible. Taggart didn’t coach in the bowl because he had taken the South Florida job. It took him three years to take the Bulls to a bowl; in his fourth year, they went 10-2. He took over the Ducks this season after Mark Helfrich was fired.

Shaw said Taggart has brought an “attacking defense’’ to Eugene, although the Ducks (4-2, 1-2 Pac-12) have been hard hit by injuries to their offense.

Surprised at departure: Shaw said he was surprised that Oregon State and head coach Gary Andersen parted ways Monday. The Beavers are 1-5 (0-3 Pac-12), but “they’ve been more physical, a lot more sound,” Shaw said. “I thought they were kind of on the right track.”

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