KANSAS CITY, Mo. -- The Kansas City Chiefs have been so bad on defense that they’ll happily accept improvements wherever they can get them. So even in a game when they allowed 177 rushing yards and played against a struggling quarterback in Denver’s Trevor Siemian, the Chiefs were encouraged after getting three interceptions and, for the first time all season, not allowing a pass play of more than 30 yards.

These developments were accompanied by a couple of lineup changes. The Chiefs replaced starting cornerback Terrance Mitchell with the seldom-used Kenneth Acker. Steven Nelson played as the nickel back for the first time this season after missing the first seven games because of core muscle surgery. Mitchell didn’t play, and Phillip Gaines, who had been the third cornerback, got in only on special teams.

Given the improvement in pass defense, the Chiefs may go with Marcus Peters, Acker and Nelson as their top three Sunday against the Cowboys in Dallas. They also may go back to Mitchell as part of the rotation.

Whatever they do, it seems certain that Nelson will be a part of things. Siemian threw a couple of passes toward Nelson on Denver’s first drive, but both were incomplete.

“I felt like early he was making plays on the ball. He had opportunities there, and he made them and took advantage of them," coach Andy Reid said. “Later in the game there weren’t quite as many opportunities, but that was just because of where the quarterback was going with the ball.

"But I thought he really played well. We have a short week that he has to bounce back from, and after his first game he’s probably going to be a little tender ... just physically sore (because) he hasn’t played in a while. He brought some energy there, too. He was fresh and a live wire out there."

During the summer, Nelson appeared primed for a big season. The Chiefs’ third-round draft pick in 2015 -- the same year they picked Peters in the first round -- Nelson established himself as a starter last season. He was one of the stars of offseason practices, but his training camp was disrupted an injury to his core abdominal area (previously called a sports hernia).

The Chiefs put Nelson on the injured reserve list before the start of the season.

“Thank God it wasn’t a knee or a back so I was able to bounce back," Nelson said. “Over time I worked really hard, and sometimes I overwork myself. I think it probably came from that.

“This year, I just know I was going to take it to another level. I had very high expectations coming into the season. I worked my tail off in (offseason practice). For something to happen like that was mind-blowing."

Coming off the injury, he was prepared for Siemian to test him.

“They thought they would get easy completions, but it didn’t happen that way," Nelson said.

Nelson had been preparing to play against the Broncos for some time. But the Chiefs’ move involving Mitchell and Acker was surprising, only in that Mitchell had been the regular while Acker had barely played.

But Mitchell, playing opposite Peters, was getting picked on by opposing quarterbacks and giving up a large share of big plays. So the Chiefs went with Acker, who had one of their three interceptions.

“Sometimes it’s OK if you have a young guy that might need to step back to take a step forward," Reid said of Mitchell. “Sometimes that happens. Nothing wrong with (Mitchell). He’s a good football player. But sometimes just to take a step back just for a minute and take a look at it from that side. We’re lucky we have him here. We thought we’d give Acker an opportunity, and he had had some snaps for us before last year and we felt like he deserved an opportunity there and did a nice job with it."