Interim head coach Jim Bates said Garrett's experience is one of the thins that made him appealing to the Dolphins.
In addition to Fiedler's injury, A.J. Feeley also is nursing a hip injury.
"I've worked personally with Jason in Dallas," Bates said Wednesday. "We felt like at this time with the situation, we needed a guy who can learn the offense in case we needed him this week. He is a very, very intelligent young man who's been there and has done it."
As for Fiedler, he was placed on injured reserve after seeing spine specialist Dr. Robert Watkins in Los Angeles.
"Obviously, I'm disappointed in the fact that I won't be able to finish the year with the team," Fiedler said in a statement released by the Dolphins.
"For my career, the best thing was to sit out the rest of the year and be ready and healthy to go next year."
As for Williams, his attorney said yesterday an agreement had been reached with the NFL that would allow Williams to rejoin the Dolphins and sit out four games this season, thus making him eligible to resume team activities immediately after the season.
The only issue now is whether the plan works for Williams, who told the San Francisco Chronicle only a few days ago he wasn't interested in playing.
If Williams does agree to the plan, it's more than likely the Dolphins will try to trade him next offseason, although at this point they probably won't be able to get much for him.