The story line for Sunday is obvious: On-fire Cardinals quarterback Carson Palmer can be deadly going deep and the 49ers secondary was just torched by long-ball-throwing Ben Roethlisberger in a 43-18 loss in Pittsburgh on Sunday.

However, Arizona has an emerging weapon beyond Palmer and a wide receiver corps that includes Larry Fitzgerald, John Brown and Michael Floyd.

Ever heard of David Johnson?

The NFL is starting to take notice of the rookie running back, a third-round pick from Northern Iowa, who has needed just eight quarters to make history: Johnson is the first player to collect a rushing, receiving and kickoff-return touchdown in his first two games.

“Any time you can do something that’s the first time in the history of the game, that’s very special,” Arizona coach Bruce Arians said today on a conference call. “He’s maximized his opportunities and maybe we want to give him a few more opportunities. He’s a big active guy.”

Indeed, Johnson (6-1, 224) has done a lot with a little, collecting 251 all-purpose yards and three touchdowns on nine touches.

In a 31-19 season-opening win against the Saints, he didn’t have a carry, but had a 43-yard kickoff return and a game-sealing 55-yard touchdown catch.

On Sunday, in a 48-23 win at Chicago, he had a game-opening 108-yard kickoff return and his five-carry, 42-yard performance included a 13-yard touchdown run.

Johnson could see more time in the backfield Sunday against the 49ers. Andre Ellington (knee) won’t play and Chris Johnson will remain the starter in his absence. Arians said Johnson is being brought along slowly because he was sidelined for much of the summer with a hamstring injury. Arians indicated Johnson hasn’t been ready to pick up blitzes in the passing game.

“He missed all of that stuff in training camp,” Arians said to Arizona reporters this week. “He’s big enough and everything, but until he takes one and gets a little bit better at it, you can only do so much if you can’t pass the ball.”

However, there appears to be little question about Johnson’s ability as a runner. Arians said his style is a “whole lot like Matt Forte,” the Bears’ Pro Bowl running back with five 1,000-yard seasons.

“They have a glide mode and it is really fast,” Arians said. “It’s deceiving.”

Twitter: @Eric_Branch