The crew assignment was officially announced November 20, 1967. For crewmates, Armstrong was assigned Lovell and Aldrin, from Gemini 12. After design and manufacturing delays of the lunar module (LM), Apollo 8 and9 swapped prime and backup crews. Based on the normal crew rotation, Armstrong would command Apollo 11, with one change: Mike Collins on the Apollo8 crew began experiencing trouble with his legs. Doctors diagnosed the problem as a bony growth between his fifth and sixth vertebrae, requiring surgery. Lovell took his place on the Apollo8 crew, and, when Collins recovered, he joined Armstrong's crew.