Diamantakos made his debut on 23 August 2014 with Olympiacos in the Superleague on a home game against Niki Volos and managed to score his first goal in the 28th minute with a close range strike.

[12] On 12 December 2015, he scored his third goal in the league, helping his club return to winning ways against SpVgg Greuther Fürth.

[15][16] On 20 February 2016, he scored the second goal helping his club to escape with a 2–2 home draw against Eintracht Braunschweig after teammate Ylli Sallahi had been sent off.

[19] On 2 May 2016, Diamantakos scored twice as Karlsruher SC beat neighbours SV Sandhausen 3–0 to record a third successive home win.

[22] On 19 November 2016, he scored the leading goal for Karlsruhe, as they dominated in the first half of the game, but lacked the force and determination in the attack, facing an unexpected 1–3 loss from SV Sandhausen.

[34] On 2 November 2019, Diamantakos scored with two penalty kicks giving a two goals-lead in a frustrated 2–2 home draw game against Karlsruher SC.

[36] On 1 March 2020, he netted with a crisp drive after an assist from Marvin Knoll in a 3–2 home win game against VfL Osnabrück.

[37] A week later, he cutting inside before smacking a shot into the roof of the net, scoring a late goal after Borys Tashchy sealing a 2–2 away draw game against SV Sandhausen.

[38] On 30 May 2020, Diamantakos scored with a left footed shot from outside the box to the bottom right corner after Bjarne Thoelke's assist in a 1–1 away draw against his ex-club Karlsruher SC.

[39] On 21 June 2020, Diamantakos used a jumble in the SSV Jahn Regensburg's defense to score his eleventh goal of the season, in a final 1–1 draw.

[44] Diamantakos' next goal came in their next match versus FC Goa on 13 November, where he netted a penalty in the closing moments of the first-half as the Blasters won 3–1 against the Goan side.

[51] He was then included in the Blasters' squad for the 2023 Indian Super Cup,[52] and on 8 April, Diamantakos captained the club in first game of the tournament against RoundGlass Punjab FC in the absence of Jessel Carneiro and Adrian Luna.

[55] After an impressive debut season with 12 goals in 24 matches, on 4 May 2023, Kerala Blasters announced that they have extended the contract of Diamantakos till 2024.

[57] Diamantakos returned to the field on 1 October, in the second league game against Jamshedpur at home, by coming as a substitute in the 60th minute, in which he assisted for Adrian Luna's goal and helped the Blasters to win the match by the score of 1–0.

[59] On 4 November, in the away match against East Bengal, Diamantakos was shown a second yellow card, and was thus sent-off in the 89th minute, for removing his shirt after scoring the second goal of the Blasters that sealed them a 1–2 victory.

[61] He would score his next goal against the newly promoted side, Punjab FC on 15 December, where he successfully converted a penalty, as it secured the Blasters a 0–1 win.