The 2007–08 season was promising for Papadopoulos. He made his competitive debut for Olympiacos in the Greek Superleague as a substitute during a 3–1 win over Atromitos on 2 December 2007, thus becoming the youngest player to ever play in the Greek league at the age of 15 years and 283 days. He also made the substitutes bench in the UEFA Champions League against Real Madrid and Lazio, and featured in several Greek Cup games. If Papadopoulos had featured in any Champions League games, he would have broken the record for the youngest player to ever compete in the competition by nearly two years.