The Dolphins signed J.P. Losman on Tuesday and placed veteran Sage Rosenfels on the reserve/non-football illness list.
"I am thankful for the opportunity the Dolphins gave me to come here and play," Rosenfels said in a statement issued through his agent, Rick Smith. "I was looking forward to contributing to the team when I signed.
"When I first joined the Dolphins, all tests showed that I was healthy and ready to go. But after practicing for a few weeks, especially in the heat, some new indications made it obvious that I need to rest and allow my body to regain all requisite strength. I want to thank the Dolphins for their understanding and wish them the best of luck the rest of the season."
The Associated Press contributed to this report.