SANTA CLARA >> Wide receiver Brandon Aiyuk sustained what the 49ers believe is a season-ending knee injury Sunday, compounding their sorrowful feelings after being dealt a 28-18 loss to the two-time defending champion Kansas City Chiefs.

“No way to sugarcoat that: We got our (butt) kicked today,” coach Kyle Shanahan said.

While Patrick Mahomes made timely plays against the 49ers’ defense similar to his two Super Bowl wins over them, counterpart Brock Purdy had three passes intercepted, the final of which came in the end zone with 9:29 remaining.

Purdy did produce a pair of 1-yard touchdown runs, but Mahomes had one himself as the Chiefs essentially controlled most of the game.

Purdy’s starting wide receivers, Aiyuk and Deebo Samuel, didn’t last the first half to further hamstring an anemic offense against the Chiefs’ shrewd scheme and powerful linemen.

Whereas Samuel briefly played though a pregame illness, Aiyuk sustained what the 49ers fear is a torn anterior cruciate ligament in his right knee. “That’s what it looks like,” Shanahan said. “We don’t know for sure but doing the tests with the trainers, that’s what we hear.”

The 49ers (3-4) host the Dallas Cowboys (3-3) next Sunday before reaching a Week 9 bye. The Chiefs improved to 6-0 this season, and coach Andy Reid improved to 22-4 coming off a regular-season bye in his career.

The 49ers’ greatest bright spot came from the mere debut of rookie wide receiver Ricky Pearsall, who 50 days earlier survived a gunshot wound through his chest in a San Francisco robbery attempt. Pearsall had three catches for 21 yards.

“It meant everything,” Pearsall said of debuting. “Obviously, with all the adversity I went through these past months, it was really good to go out there with my guys again.”

“We celebrated that (return) almost all week with him. He was going to get eased in today,” Shanahan said. “That changed a little when Deebo went out, and it changed a bunch when Aiyuk went out.”

Aiyuk got injured once hit by defensive backs Trent McDuffie and Chamarri Conner at the Chiefs’ 14-yard line, the price for making a 15-yard catch that set up an Anders Carlson field goal for a 14-6 halftime deficit.

Thus, the 49ers’ top three wide receivers were unavailable for a second-half comeback bid: Aiyuk was in the locker room needing crutches and a knee brace; Samuel was out of uniform on the sideline after halftime; and Jauan Jennings did not suit up at all because of a hip injury that isn’t considered long term.

Said George Kittle: “I look in the huddle and I’m standing next to Ricky, Jake (Cowing), Chris Conley and J.P. Mason. It’s a lot different than Deebo, Jauan, Aiyuk and Christian McCaffrey. So, a little different.”

Two series before Aiyuk’s injury, he dropped a third-down pass at the Chiefs’ 40-yard line, eliciting boos from the home crowd. The next series ended with a third-down overthrow toward Aiyuk. He finished with two catches on six targets for 23 yards. He’s yet to register a touchdown pass since signing a four-year, $130 million deal in late August.

Sunday’s receiver issues surfaced further in the third quarter when Purdy’s wayward pass toward Ronnie Bell got intercepted at the Chiefs’ 25-yard line by Chris Roland-Wallace. Purdy’s more damning interception came when his arm got hit by George Karlaftis (who beat right tackle Colton McKivitz) and landed in Jaden Hicks’ enemy hands in the end zone, with the 49ers down 21-12.

Both Purdy and Mahomes posted their worst passer ratings as a starter. Purdy was 17-of-31 for 212 yards and a 36.7 passer rating; he had a 30.1 mark two years ago when he attempted his first career throws in a loss to the Chiefs.

Mahomes’ 44.4 passer rating came by completing 16-of-27 passes for 154 yards with interceptions snagged by defensive tackle Kalia Davis and cornerback Deommodore Lenoir.

As Davis said, however: “He did some magic out there today, and we need to clean stuff up.”

Mahomes’ rushing ability, which converted a fourth-down keeper on their Super Bowl-winning drive eight months ago, helped the Chiefs pull away for critical points in the fourth quarter. Mahomes scored on a fourth-and-goal keeper from the 1, and he punctuated that touchdown by throwing a left shoulder into safety Malik Mustapha.

Earlier on that drive, Mahomes created a career-long 33-yard run by racing down the left sideline past linebackers Fred Warner and Dee Winters.

“They find ways to win,” Warner said. “That’s what’s great about their team. It doesn’t always have to be him making great plays. Their team won the game today.”

Lenoir returned his interception to the 23-yard line to set the stage for a touchdown drive that ended with a 1-yard sneak by Purdy, who three snaps earlier converted on another 1-yard sneak. That had pulled the 49ers within 14-12, and rather than go for a tying 2-point conversion, replacement kicker Anders Carlson went in and missed a point-after kick off the left post.

The first-half scoring summary: Kansas City scored two touchdowns (on Kareem Hunt runs), and the 49ers scored on two field goals (by Carlson, their third kicker in as many games).

Kittle finished with a team-high 92 yards on six catches, and Cowing’s first two career catches went for 51 yards.

“You obviously want to beat a team that’s had your number since I’ve been here, but we didn’t play good enough,” said Nick Bosa, who shared a sack with Leonard Floyd, registered three hits on Mahomes, and came away with a bruised left elbow.

Now the 49ers hope to repeat what they did in 2021 and ‘22 — overcome a 3-4 start to make another deep playoff run.

“We’ve been through a lot of tough losses, a lot of tough season starts,” Bosa added. “But it’s all about getting into the playoffs, and you’ve got to do that by winning enough games, winning your division, and that’s where our focus is.”