Bruce Arians plans to keep his running backs guessing this season. Will they be in single-back formation? Will there be more two-backs sets as he has indicated and shown in camp? Will they line up as receivers?

"I have no idea," top back Andre Ellington said, grinning. "I just come here, see the script, see what plays I have for the day and try to execute those."

Some national analysts have questioned the Cardinals' 2014 backfield, both for its lack of a true No. 1 back -- the Cardinals prefer Ellington (5-9, 199) in space as opposed to between the tackles -- and for its lack of established performers. The top three backs on the Cardinals' depth chart -- Ellington (652), Stepfan Taylor (115) and Jonathan Dwyer (197) -- didn't combine for 1,000 yards rushing last season.

Arians probably wouldn't turn down an Adrian Peterson, LeSean McCoy or Matt Forte if they came his way, but a feature back is not what this offense is about now that Rashard Mendenhall is gone, and it's not where the NFL is trending. Only a handful of running backs have averaged 20-plus rushing attempts each of the past six seasons.

Rules changes that benefit the passing game have helped push offenses in that direction, but so has an emphasis on speed in defenses that makes running the ball more difficult.

So when evaluating the Cardinals' backfield, it's helpful to evaluate the total package. As a rookie last season, Ellington finished 14th among NFL running backs in receiving yards (371), and he had fewer receptions than any of the players who finished ahead of him.

That figures to change this season. In the offseason, Ellington learned all three receiver positions, marking the first time in his career he lined up at an offensive position other than running back.