Raiders get help from another undrafted rookie in James...

When the Raiders got back to the locker room after their 38-24 win, they were all feeling good. But Derek Carr and Sebastian Janikowski were congratulating one teammate in particular: James Cowser.

Who?

Exactly.

The linebacker/defensive end is another undrafted free-agent rookie contributing to this surprising Oakland season. Activated last week off the practice squad to take the place of injured third-round pick Shilique Calhoun, Cowser made an impact Sunday. In the second quarter, he got to Buffalo’s Tyrod Taylor for a 10-yard sack.

It was a huge play at the time because the Bills were getting in an offensive rhythm, and the sack helped force Buffalo’s first punt of the game.

Cowser went undrafted out of Southern Utah, despite setting an FCS record with 43.5 sacks and setting a Big Sky Conference record of 80 career tackles for a loss.

He might have been downgraded because of his size (6-foot-3) and his age. Now 25, he spent a year in China during college on a mission for the Mormon church.

“Not getting drafted was a bummer for about five minutes,” Cowser said this summer. “If you’re dealt a bad hand, that’s what you’re going to have to play, anyway, so make the most of it. I couldn’t be happier being on the Raiders.”

The Raiders were one of 15 teams that contacted Cowser after the draft. Cowser signed with Oakland because, he said, he hit it off best with the team.

The Raiders have shown a great knack for finding diamonds in the rough, a sign that their talent evaluators know what they’re doing.

“To have a late-round or undrafted guy produce, that’s what it’s all about,” general manager Reggie McKenzie told reporters last week.

Another undrafted rookie free agent who continues to make an impact is running back and return man Jalen Richard. Richard rushed for 53 yards Sunday. He gives the Raiders an effective one-two rushing punch with Latavius Murray.

Late in the third quarter, trailing 24-16, Richard fielded a short punt and returned it 17 yards to the Bills’ 38. On second down, he scooted 21 yards, setting up the Raiders’ second touchdown and helping channel the momentum.

“It’s just testament to this organization and the staff upstairs to find guys that are somewhat overlooked,” Richard said. “They give those guys a chance, and they turn out to be good players and give depth to the team.”

Ann Killion is a San Francisco Chronicle columnist.