But at least the Colts gave themselves a chance as running back Frank Gore scored two touchdowns at Sun Life Stadium.
It was a happy homecoming for Gore, a former Miami Hurricanes star. He is in his 11th year in the NFL but had never played a pro game in his hometown before Sunday.
"I'm happy to be back," said Gore, who ran 15 times for 85 yards after failing to clear 60 yards in his past five games. "It's been a tough year. I just wanted to let my fans remember what I've been doing since high school. I wanted to let them know I can still play."
The Colts (7-8) are still alive in their bid to win the AFC South division ... but barely. The Colts need a win at home next week against the Tennessee Titans, a Houston Texans loss when they host the Jacksonville Jaguars and then more help in terms of the tiebreakers. The Texans are 8-7.
Miami (5-10), which was eliminated from playoff contention weeks ago, had a chance to win on Sunday when they earned a first and goal on the Colts five-yard line with 40 seconds left.
But Dolphins quarterback Ryan Tannehill fired three incomplete passes and then was sacked by defensive end Robert Mathis on fourth down, effectively ending the game.
The Colts, who produced six sacks, took advantage of confusion on the Dolphins offensive line on that final play, particularly at center, where rookie Jamil Douglas was filling in for Pro Bowl center Mike Pouncey. It appeared that Douglas snapped the ball too early on the play.
"He feels like it's on his shoulders, and it's not," Dolphins interim coach Dan Campbell said of Douglas. "Ultimately, we're all responsible. It's never one man's fault."
The Colts won despite some adversity at a crucial position. Already without injured starting quarterback Andrew Luck, Indianapolis lost backup Matt Hasselbeck with 5:33 left, and Indianapolis leading 8-6.
Charlie Whitehurst came in and played the rest of the game at quarterback, passing for just 78 yards but avoiding turnovers.
"He managed the game well," Colts coach Chuck Pagano said of Whitehurst. "He came up with a huge third-down play (in the fourth quarter). It looked like he threw the ball up for grabs, but he knew exactly where he was going with the football (tight end Jack Doyle)
"Charlie is a tough son of a gun."
The Dolphins, who have lost three straight games and seven of their past nine, were led by wide receiver Jarvis Landry, who caught seven passes for 111 yards.
Landry, the Dolphins single-season leader in receptions, went over 100 catches for the year.
Indianapolis scored first, getting a safety with 9:16 left in the first quarter on defensive tackle T.Y. McGill, who sacked Tannehill.
The Colts extended their lead to 8-0 on a 37-yard touchdown run by Gore, who broke two tackles as he burst through the middle of Miami's defense. The extra point failed.
Miami blew a chance to score in the final minute of the first quarter as a Tannehill red-zone pass was intercepted by cornerback Vontae Davis, a former Dolphin.
The Dolphins scored on a one-yard touchdown run by Lamar Miller, who capped an 11-play, 69-yard drive. The Dolphins went for two points and a tie, but Tannehill's pass was incomplete.
Two plays after Hasselbeck left the game, Indianapolis went up 15-6 on Gore's 11-yard run.
Miami and Indianapolis traded field goals -- 33 yards by the Dolphins' Andrew Franks and 38 yards by the Colts' Adam Vinatieri. That left the Colts with an 18-9 lead with 8:30 left to play in the fourth quarter.
Franks booted another field goal, this one from 20 yards, to cut the Colts cut to 18-12 with 5:17 left.
On their final possession, the Dolphins took over at their own 20 with 2:48 remaining. It took Miami eight plays to get to the Indianapolis five-yard line.
But then came three straight incomplete passes. On fourth down, Tannehill said his snap count backfired.
"We had a double cadence," Tannehill said. "We had a cadence issue. We give them a hard cadence to get them to jump (off-sides) or to see what (defensive alignment) they're in, but it didn't work out for us."
NOTES: With QB Andrew Luck missing his sixth straight game, the Colts signed former Miami Hurricanes QB Stephen Morris, 23, as a backup. ... Rookie CB D'John Smith, the Colts' third-round pick, was placed on injured reserve, spoiling his homecoming (he played locally at Florida Atlantic). ... The Dolphins were without six starters: DT Cam Wake (foot), DT Earl Mitchell (calf), LB Koa Misi (back), C Mike Pouncey (left foot), RT Ja'Wuan James (toe) and WR Rishard Matthews (ribs), . ... Pouncey is the first Dolphins offensive lineman since Keith Sims 20 years ago to make three straight Pro Bowls. But two big-name Dolphins were snubbed on Tuesday when the Pro Bowl selections were announced: S Reshad Jones and DT Ndamukong Suh. ... Miami's 31-year Super Bowl-appearance drought is the sixth longest in the NFL.