Agathe made his debut for Celtic on 14 October 2000 in a home league game against St Mirren. Fielded by O'Neill wide on the right, Agathe turned in an outstanding performance in a 2–0 win for Celtic, with his pace and skill running the St Mirren defence ragged. O'Neill enthused about Agathe's debut, "He was absolutely brilliant, fantastic even. I did not think he would last more than 60 minutes but he has so much strength and pace". Agathe kept his place in the team, playing at right wingback, and scored his first goal for Celtic in December 2000 against Dundee with a header in the last minute to give his team a 2–1 win. He went to help Celtic win a domestic treble of League, Scottish Cup and Scottish League Cup that season. Agathe provided the assist for Jackie McNamara's opening goal in their 3–0 win over Hibs in the Scottish Cup Final. However, he was cup-tied for their victorious 2000–01 Scottish League Cup campaign.