Dolphins quarterback Ryan Tannehill (17) could benefit greatly by having former Bronco Knowshon Moreno (28) in the Miami backfield. (J Pat Carter/Associated Press)

MIAMI GARDENS, Fla. — The Miami Dolphins need to give their third-year quarterback, Ryan Tannehill, some help on offense this season if he’s going to develop into a top-tier NFL passer and make the team a legitimate contender for the AFC playoffs. There simply wasn’t enough assistance for Tannehill last season, as the Miami offensive line allowed him to be sacked 58 times and the Dolphins had the NFL’s 26th-ranked rushing offense.

So even on a night when the starting offense produced only a single field goal while playing into the third quarter of the third preseason game, there was an encouraging sign for the Dolphins here Saturday against the Dallas Cowboys. That was the running of tailback Knowshon Moreno, who rushed for 64 yards on 10 hard-charging first-half carries in his preseason debut for the Dolphins after offseason knee surgery.

“I thought he ran the ball well,” Dolphins Coach Joe Philbin said at his postgame news conference late Saturday night. “I thought he ran the ball physically. His vision looked like it was pretty good. He broke a couple tackles, which was a positive thing.”

The Dolphins signed Moreno as a free agent after he ran for 1,038 yards and 10 touchdowns last season for the Denver Broncos. He also had 60 catches for 548 yards and three touchdowns as a key contributor to Denver’s high-powered offense. But Moreno underwent arthroscopic knee surgery during the offseason and has not overtaken incumbent starter Lamar Miller as the Dolphins’ No. 1 tailback, at least not to this point.

Tannehill, the third quarterback selected in the celebrated 2012 draft class behind Andrew Luck and Robert Griffin III, did his part last season, making significant improvements from his rookie-year performance. He threw for 3,913 yards and 24 touchdowns with 17 interceptions, after managing only 12 touchdowns and 3,294 yards to go with 13 interceptions as a rookie in 2012.

Tannehill’s growth as a quarterback nearly helped the Dolphins make the playoffs. They managed to have an 8-6 record with two games left, even amid the controversy of their locker room bullying scandal. But they lost their final two games and finished behind the San Diego Chargers, who went 9-7, for the second wild-card spot in the AFC.

Expectations for Tannehill and the Dolphins are high this season. The New England Patriots have upgraded their secondary and could have better receiving options for quarterback Tom Brady as he chases a fourth career Super Bowl triumph. They appear to be the clear favorites in the AFC East. But the Dolphins, who host the Patriots in their Sept. 7 regular season opener, could be New England’s top challenger in the division, and securing a spot in the AFC’s postseason field certainly is not out of the question.

The offense will have to perform better overall than it did in Saturday’s primary preseason tuneup for the regular season. Tannehill missed an open receiver or two and threw an interception early in the third quarter before he and the other offensive starters exited the game.

“I thought we’d score more points, quite frankly,” Philbin said. “I didn’t think we’d end up with three points when we took the first offensive unit out in the beginning of the third quarter. So I was expecting more production from those guys from a points standpoint. … We moved the ball well. But at the end of the game, you’ve got to have points on the board and I didn’t think we did that well enough.”