Garner, who started eight games at guard last season and was backing up right tackle Vernon Carey this preseason, broke the foot in practice last week, according to The South Florida Sun Sentinel.
Garner was at his more natural right guard spot when he re-injured his foot, according to the Herald.
Garner's injury complicates things for Sparano, who would like to establish his starting guards before the Dolphins' third preseason game, Aug. 27 against the Atlanta Falcons. Rookie guard John Jerry, a third-round draft pick who had been starting at right guard, injured his right knee Monday and didn't train Tuesday, the Sun Sentinel reported.
"We'll keep slugging it out here, but I think it has to settle down by the next week, or by the end of this preseason game," Sparano told the Sun Sentinel. "I kind of got to get it settled down one way or the other.
"It is unmuddying a little bit with the players we have right now in some situations. I'm starting to see a couple of guys getting a little bit better. Unfortunately, what I thought was happening in (Saturday's 10-7 victory over the Tampa Bay Buccaneers), we came out here in practice, a few of them took a step backwards in practice ... but today was a little bit better."