Cardinals running back Chris Johnson breaks through the line against the Saints Sunday.

There was a reason while David Johnson was the Cardinals' pick in the third round in April's draft, and why the Cards chased veteran Chris Johnson even before injuries cropped up early in training camp.

Once the Cardinals lost Andre Ellington (to injury) and Jonathan Dwyer (to arrest) last season, the running game took the gut punch. Even with Ellington healthy coming into 2015, the Cards weren't going to be caught again short on running backs.

"We wanted depth in that room," coach Bruce Arians said Monday, as he acknowledged Ellington had suffered a "mild" sprain of the posterior cruciate ligament in his right knee. "We have it. We don't want to have to use it, but hopefully it's a very short-term

thing."

The Cards figure to have the Johnson and Johnson show for now. Arians wouldn't put a timetable on Ellington's return, even going as far as saying Ellington could still play Sunday in Chicago. But that would seem to be unlikely – especially since, unlike last year, the Cardinals don't need to rush anyone back.

That's the point of the depth.

Chris Johnson, who had 37 yards on 10 carries against the Saints, slides into the starting lineup. David Johnson, who didn't have an offensive touch other than the swing pass he caught with less than two minutes left that he turned into a 55-yard touchdown, is the backup.

"That's always the possibility playing running back," Chris Johnson said Monday. "We get hit every single play. We probably play the most physical position on the team. You have to have other guys who can step in and take care of that role because you never know."

The Cardinals also have veteran Stepfan Taylor if needed. Arians said even if Ellington can't play, he was comfortable with just three active running backs and didn't necessarily see the need to promote Kerwynn Williams from the practice squad.

(Arians could also use backup offensive lineman as a blocking back in short-yardage, if needed.)

The Cardinals ran for 120 yards on 25 attempts Sunday, a solid average of 4.8 yards a carry. That included a 5.8-per-carry number from Ellington, who looked like the Ellington of 2013 before getting hurt. Arians called him "explosive," although he said the running game left some 30 yards on the field because of unfinished blocks or missed holes.

Arians said Johnson was "decisive" in his time on the field. Arians dismissed the idea Chris Johnson could usurp Ellington as starter permanently – "It will always be a one-two punch," he said – but was fairly clear that the veteran Johnson would stay ahead of the rookie Johnson for the time being.

"I don't want to put too much pressure or take rookies and give them too much of a piece of the pie too early," Arians said. "I want them to earn it. It's not healthy to put them in that situation too much. They'll earn it."

Regardless, David Johnson said he was "definitely ready" to step in for Ellington if asked.

David Johnson is the better receiver, and Arians acknowledged he would use Chris Johnson differently in the passing game compared to David Johnson or Ellington. Chris Johnson also has more experience as a pass blocker.

"Andre is so used to this offense and Chris hasn't been in a system like this," quarterback Carson Palmer said. "But he's picked it up so fast and picked up the protections. He's just been a true vet and he's come in and studied it and known the protections, really from the first walkthrough it seemed."

Now the Cardinals need it with Ellington probably on the shelf.

"I think both those guys can pick up that slack easily and are more than qualified," Arians said.