Tannehill completed 14 of 21 passes for 167 yards, including an 11-yard touchdown pass to tight end Charles Clay in the third quarter for Miami's only score in a 20-7 loss to the Tampa Bay Buccaneers on Friday.
David Garrard, who was listed first on the depth chart, is injured. His agent, Al Irby, confirmed Friday that Garrard will undergo knee surgery and miss up to a month.
Tannehill could be closing the gap on Matt Moore, who started the last 12 games last year for Miami (No. 27 in the AP Pro32) and was second on the depth chart behind Garrard.
Despite Moore's edge in experience, first-year Dolphins coach Joe Philbin said the gap between Moore and Tannehill wasn't very big.
''It's relatively close, probably,'' Philbin said.
Moore was impressed with Tannehill.
''The kid went out there and moved the ball and put us in the end zone when we were in the red zone,'' Moore said. ''I know he's good and he's talented and he's got the build and mindset you want in a quarterback. I just need to focus on my job and getting better and the rest will take care of itself.''
Tannehill, taken eighth overall in the draft, is the Dolphins' quarterback of the future, but he's hoping to convince his coaches to make him a starter right away.
He had a strong start on Friday night.
''He's poised,'' Clay said. ''I was surprised at how calm he was. He was in the huddle, laughing and joking around at times. Coming in as a rookie, having his first game, you could just tell he was out there having fun.''
Before his TD pass, a strike down the middle, Tannehill had a touchdown pass to wide receiver Roberto Wallace late in the second quarter overturned by instant replay.
Two plays later, Tannehill missed Clay near the sideline on a fourth-and-goal from the 3-yard line.
He was 8 for 9 on his touchdown drive, the highlight was a nice touch pass to Wallace over a linebacker for 19 yards on third-and-11.
The only incompletion on the drive was a pass that was batted down at the line of scrimmage.
Tannehill, who finished the game with a 106.6 passer rating, entered with 3:16 left in the second quarter to the applause of a sparse crowd still sitting despite a driving rain. His first pass, on third-and-6, was thrown behind wide receiver Julius Pruitt and led to a three-and-out.
''I just have to go out and play my best,'' Tannehill said. ''It's small steps and just trying to be the best I can be. At the end of the camp, it's up to the coaches.''
Philbin declined comment on Garrard's injury.
Along with Garrard, Dolphins starting linebacker Kevin Burnett also missed Friday night's game despite taking part in warmups. Backup offensive lineman Nate Garner was helped off the field in the fourth quarter after sustaining an ankle injury.
Moore got the start in Garrard's place and completed 7 of 12 passes for 79 yards with one interception for a rating of 43.4.
Moore had some tough luck. He produced three first downs on his first drive, only to see Chad Johnson let a pass go through his hands on third-and-3 from the Tampa Bay 33. It was the only pass thrown in Johnson's direction.
''You've got to make those catches and move those chains,'' Philbin said. ''That's a perfect illustration of why the outcome ended up as what it was.''
Moore's final pass midway through the second quarter was intercepted by rookie linebacker Lavonte David, but the ball was tipped at the line of scrimmage by defensive tackle Frank Okam.
''For the most part, I thought it went pretty smooth,'' Moore said. '' I thought we moved the ball well. We just need to tighten down some things. I've just got to focus on getting better day in and day out. With the reps I get, I've got to do well.''
The Dolphins' second-round pick of the 2012 draft, tackle Jonathan Martin, also got extended playing time but had an up-and-down performance.
Martin started at right tackle and played into the third quarter, long after the other starting offensive linemen left the game. Martin held up well in pass protection but also was flagged for two false starts.
For Tampa Bay (No. 27), the game was a good start under new head coach Greg Schiano.
The Bucs looked sharp offensively at the start of the game, scoring touchdowns on their first two possessions to take a 14-0 lead.
''We put a lot into it all the way, dating back to way back in the spring,'' defensive tackle Gerald McCoy said. ''It was our first chance to showcase it. I think we did a fairly decent job.''
Starting quarterback Josh Freeman left the game after playing only one series, completing 4 of 5 passes for 41 yards. LeGarrette Blount finished off the drive with a 1-yard touchdown run on fourth-and-goal.
Bucs first-round pick Doug Martin scored on a 2-yard run on Tampa Bay's next drive and finished with 21 yards on seven carries.
David, the Bucs' second-round pick, had a nice preseason debut in his hometown. In addition to his interception, he also had a tackle for loss and a special teams tackle.
''He's done a real good job since we've started and I think he continued that tonight,'' Schiano said. ''He flew around out there. He looked comfortable, which is great. And for him to do it in his hometown is fun, too.''
Tampa Bay's third-quarter field goal was set up by a 74-yard kickoff return by rookie seventh-round pick Michael Smith, a running back from Utah State.
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