Tramaine Brock is the 49ers’ best cornerback.

So why didn’t he shadow one of the NFL best wide receivers, Pittsburgh’s Antonio Brown, in a 43-18 loss to the Steelers on Sunday?

Defensive coordinator Eric Mangini was posed that question after Brown picked on cornerback Kenneth Acker, who was making his second career start, throughout his nine-catch, 195-yard, one-touchdown performance.

Mangini said part of the reason was that the 49ers wanted to give quarterback Ben Roethlisberger a variety of defensive looks.

“Really with any team, especially with a quarterback like Ben, experienced guys, you can’t just have one way,” Mangini said. “You’ve got to have different ways to try to mix it up and make sure it’s not just the same thing over and over again.”

Acker allowed six catches, all to Brown, for 120 yards, according to Pro Football Focus.

“If you look at the whole game in general, it was about four plays that took a toll on us,” Acker said. “You look at like four plays can’t kill you, but four plays can definitely make a difference in a game. So we just try to make sure that those big mistakes never happen again.”

Growing pains: In May, the 49ers made Clemson’s Bradley Pinion the first punter selected when they drafted him in the fifth round.

In June, they traded punter Andy Lee, 33, who ranks fifth in NFL history in net average, to the Browns and Pinion’s roster spot was cemented.

Now, in September, the 49ers are hoping their 21-year-old punter quickly emerges from a sluggish start.

“Young players are going to go through those growing pains,” special teams coach Thomas McGaughey said. “Bradley has done a really good job of kicking off. That’s truly a strength of his, and that’s why we drafted him. He’s been inconsistent with his punting as far as hitting that consistent ball all the time.”

Pinion ranks 24th in the NFL in gross average (44.4 yards per punt) and 15th in net (40.9). Meanwhile, Lee, a three-time All-Pro, ranks second in gross (55.0) and third in net (45.8).

The strong-legged Pinion is also handling kickoffs, which was placekicker Phil Dawson’s job the previous two seasons. Pinion has clearly upgraded that department: The 49ers rank fourth in the NFL in kickoff touchback percentage (88.9) after ranking 24th (43.8) and 26th (38.3), respectively, the past two years.

Injury report: Running back Reggie Bush was able to play table tennis in the locker room, but the calf injury he suffered in Week 1 kept him off the practice field for another day.

Bush, who worked on a side field with a trainer at the start of practice, was the only player who didn’t practice. Tight end Vance McDonald, who wore a brace on his right knee, and wide receiver Bruce Ellington (ankle) were limited.

Eric Branch is a San Francisco Chronicle staff writer.