There's a reason it's called a "monster."

When you have three running backs of near-equal skill but with varying skill-sets rotating in and out of an offense, the net effect can be a frightening one for a defense.

On Saturday night the Eagles gave a glimpse of what their three-headed monster might look like with DeMarco Murray, Ryan Mathews, and Darren Sproles sharing the load. And it was an effective beast - if not yet scary - in the 40-17 preseason win over the Ravens.

Chip Kelly's one-quarter teaser hinted at myriad possibilities within his explosive offense with Murray, Mathews, and Sproles splitting snaps and sometimes even being on the field at the same time.

The Eagles ran 19 plays, including three penalties, during two series with their first-team offense, and 14 of the plays involved one of the three running backs. Both drives resulted in rushing touchdowns - a 14-yard scurry by Mathews and a 2-yard tiptoe into the end zone by Murray.

"It's going to be hard for teams to focus on just one running back," Murray said. "I think it's going to be good that we're all fresh going in and out of the game."

Sam Bradford, in his first game for the Eagles, was at quarterback for the first possession, while Mark Sanchez took over for the second. Kelly pulled Bradford after an 11-play, 84-yard drive in which he took two body blows.

Bradford got up from both hits, and he threw the ball well considering it was his first game in nearly a year. But one idea behind adding both Murray and Mathews in the offseason was that getting back to a run-based scheme would limit the chances of the quarterback's reinjuring his knee.

The primary motive, obviously, was to add two talented, proven running backs after LeSean McCoy was traded. Both have extensive injury histories, and Murray is coming off a season in which he touched the ball nearly 500 times. But Kelly said spreading the playing time out between the three backs would keep everyone fresh and curb the chances of further injury.

Murray had been conspicuously absent at various points throughout the first few weeks of training camp. Kelly said he held him back in last week's preseason opener as a precaution after last year's workload, but Murray had been dealing with a hamstring injury, NFL sources said.

It was difficult to say with certainly whether Murray practiced in full this past week during scrimmages with the Ravens. He took only a fraction of the first-team snaps. But that's because he's going to take only a portion of the snaps during the season. That's the offense.

"DeMarco showed a little pop today," Kelly said. "You watched him come out the other side. I was happy with all three of those guys."

Kelly has said he wants a multiheaded backfield like the one he had at Oregon in 2011 with LaMichael James, Kenjon Barner, and De'Anthony Thomas. Murray should take on the James role as the lead tailback, but Mathews - as Barner - should get about 30-40 percent fewer carries. And Sproles should adopt the Thomas role as a hybrid tailback-receiver.

It's a small sample, but that's essentially how the numbers played out against Baltimore. Murray started and rushed five times for 17 yards and a touchdown. Mathews finished with three carries for 19 yards and a score. Sproles ran twice for 10 yards and caught two passes for 15.

Sproles' first reception converted a third down. He was lined up in the slot with Murray in the backfield and caught Bradford's throw over the middle when he beat the linebacker. Defenses are going to be scrambling if they can't get a defensive back on Sproles in pass coverage.

There could be a lot more of Sproles in the slot this season. Kelly's five best skill-position players would be either Murray or Mathews at running back, Zach Ertz at tight end, Jordan Matthews and Nelson Agholor as the outside receivers, and Sproles as the inside receiver.

That is a potent mix. He could try to get Murray and Mathews on the field together, but that is unlikely. And why would he need to?

Late in the first drive, after Murray took the bulk of the snaps, Mathews entered the game on first down at the Ravens 14. The Eagles went no-huddle the entire series. Baltimore was reeling. Mathews got the inside zone-read handoff and, behind blocking from center Jason Kelce and left tackle Jason Peters, scooted virtually untouched into the end zone.

On the Eagles' next possession, Murray caught a 14-yard pass, Sproles a 7-yard one, and then Mathews came into the game. He rushed for 2 yards and then 3 yards for a first down at the Ravens 2. Murray returned, got the tote, and actually danced into the end zone without a Raven laying a glove on him.

It was a trailer for the season, a sneak peek at the monster.

The feature-length version opens Sept. 14 in Atlanta. The Eagles are hoping it runs all season and into the playoffs. The horror would be another injury to one of the tailbacks.

@Jeff_McLane