Complex sentence: In the summer of 1999, at the age of 19, Diamantidis began his professional career when he transferred to Iraklis Thessaloniki and became a starting player there by his second season.
Split into simpler sentences: In the summer of 1999, at the age of 19, Diamantidis began his professional career, when he transferred to Iraklis Thessaloniki. He became a starting player there by his second season.