Smiley missed the last four games last year after sustaining an injury to his right ankle and leg in a game at St. Louis.
"Justin's done a good job while he's been in there," said Dolphins coach Tony Sparano. "If he doesn't play in the game, the next guy is going to have to be ready to go."
Murtha lined up with the first-team offense on Wednesday, while Garner took that spot Thursday.
Both came into the league as offensive tackles.
Along with Smiley, other players sitting out practice for a second consecutive day were starting outside linebacker Joey Porter (knee), backup nose tackle Paul Soliai (ankle) and special teams contributor Erik Walden (hamstring).
Porter has been bothered for most of the season by a hamstring injury, which forced him to sit out a game against Buffalo on Oct. 4.
If he can't play against Tampa Bay, his place in the starting lineup figures to be taken by Charlie Anderson. The Dolphins could also move Jason Taylor from strongside linebacker to weakside linebacker, and insert Matt Roth into the starting lineup.
Roth started every game at the position last season but has been coming off the bench in 2009 after missing the first six games while on the Non-Football Injury list.
While the Dolphins could be down two starters on Sunday, they'll welcome back two others who missed last weekend's game at New England because of injuries.
Linebacker Channing Crowder (shoulder) and nose tackle Jason Ferguson (elbow) both have practiced Wednesday and Thursday and they don't appear on the Dolphins' injury report. Crowder also missed the Nov. 1 game against the Jets.