Complex sentence: His pro career began with the Greek League club Olympiacos in 2013, when he signed a 5-year deal with Olympiacos, in late August 2013, which was his first pro deal.
Split into simpler sentences: His pro career began with the Greek League club Olympiacos in 2013. He signed a 5-year deal with Olympiacos, in late August 2013, which was his first pro deal.