How should fresh lamb be thawed?

Fresh lamb may be thawed in one of three ways: in the refrigerator, in cold water, or in the microwave. Never defrost fresh lamb at room temperature. Surface areas of the meat will thaw at a quicker rate than interior areas, leaving them vulnerable to bacterial growth once they reach a temperature of 40°F. 

• Refrigerator: Once whole-muscle lamb defrosts, it can remain in the refrigerator for 3-5 days if it was frozen immediately upon purchase. Ground lamb should be consumed within 1-2 days. In general, ground lamb may defrost within a day. Bone-in parts and whole roasts may require 2 days or longer.  If you decide not to use the lamb, you can  safely refreeze it within 3-5 days if it has not been thawed to a temperature greater than 40°F.

• Cold water: Do not remove packaging when defrosting lamb in cold water. Be sure the package is airtight or put it into a leak-proof bag. Fully submerge the lamb and change the water every 30 minutes, so that it remains cold. When submerged in cold water, small packages of lamb may defrost in an hour or less. A 3-4-pound roast may require 2-3 hours.

• Microwave: Plan to cook the lamb immediately after defrosting it, as some areas may become warm and begin cooking during microwaving. Holding partially cooked food isn't recommended because any bacteria present may still be active. 

Never refreeze lamb that has thawed to a temperature greater than 40°F. Freezing will inactivate but not kill any bacteria that may have accumulated as a result of thawing lamb to a temperature of 40°F or greater. If lamb has thawed to a temperature greater than 40°, it should be cooked immediately.
 
Source(s):
USDA Fact Sheet "Lamb From Farm to Table"