One of the most impactful moments of Stanford’s 21-6 loss to No. 16 Oregon on Saturday came on what was a seemingly innocuous first-quarter pass play.

On 1st-and-10 from the Ducks’ 17, quarterback K.J. Costello hit his thumb on a lineman’s helmet while throwing awry and spent much of the rest of the day trying to shake the pain out of his hand.

“I mean, I can’t catch a break right now,” said Costello, who missed the second half of the season-opening win against Northwestern and the Week 2 loss at USC with concussion symptoms. “… There’s no legitimate excuse in that scenario. I could grip it enough to throw it. …

“We’ve got guys playing banged up all over, so I feel like I’ve got to do my part.”

Costello, who is coming off a season in which he passed for 3,540 yards and 29 touchdowns, clearly wasn’t himself against Oregon. He went 16-for-30 for 120 yards and an interception and uncharacteristically missed some open receivers.

He said he stayed after practices this week and felt he was getting into a great rhythm with his receivers after missing a lot of the offseason following an ankle arthroscopy and arm troubles.

Those feelings seemed warranted on the Cardinal’s first drive as Costello marched them 61 yards on 12 plays. Then he injured his hand.

“Boom, first drive,” Costello said. “That’s not fun, but what are you going to do?”

Freshman starts: Freshman defensive back Kyu Blu Kelly made his first start, joining several other Cardinal who have made their debut starts through the first four weeks.

The others are offensive lineman Branson Bragg, offensive lineman Henry Hattis, fullback Houston Heimuli, defensive back Jonathan McGill, quarterback Davis Mills, defensive back J.J. Parson, linebacker Andrew Pryts, linebacker Curtis Robinson, offensive lineman Walter Rouse, offensive lineman Foster Sarell and receiver Osiris St. Brown.

Kelly had four tackles, including one for a loss, and a quarterback hurry.

What a rush: Cameron Scarlett tied his career high with 97 rushing yards on 19 carries, an especially meaningful feat against Oregon. The senior running back is from Portland and has remained close with many of the Ducks’ players since graduating from Central Catholic High.

Just for kicks: Jet Toner was 2-for-2 on field goal attempts to move into a tie for seventh on Stanford’s all-time list with 42 makes. He joined Conrad Ukropina (2013-16) and needs two more to match Rod Garcia’s 44 from 1971-73.

Rusty Simmons is a San Francisco Chronicle staff writer.