Complex sentence: In early July 2012 Alonso left the Red Devils because of a heart condition, subsequently retiring from football--his doctor advised him not to play with Toluca as the high altitude of the city might worsen his condition.
Split into simpler sentences: In early July 2012 Alonso left the Red Devils because of a heart condition, subsequently retiring from football. His doctor advised him not to play with Toluca as the high altitude of the city might worsen his condition.