Olympiacos were plagued with injuries ahead of their UEFA Champions League encounter with Marseille and coach Ernesto Valverde decided to field Maniatis as a defensive midfielder in a tough away game of great importance.

He has proven to be a revelation at Olympiacos where he has shown great tackling skills, superb stamina and a surprisingly good passing range with impressive vision.

[6][7][8] It was one of the most stable and most valuable players of Olympiakos during the last seasons, and in any case, the 27-year-old cashed his offer champions Greece with a new four-year contract which will tie him with the club till 2017.

[10] Olympiakos's captain Giannis Maniatis on Friday extended his contract with the Greek champions by two years despite being ruled out for four months with a knee injury.

Despite rumours which linked him with a move abroad in the past, Maniatis always had the desire to remain at Olympiakos and he was more than happy to commit his future to the club for the next four years.

[12] On 18 December 2015, almost nine months since his severe injury Maniatis, has fully recovered but he could not be able take playing time and according to Greek newspaper Fos, Olympiakos manager Marco Silva will possibly make roster cuts in January and he is in danger.

[13] [14] After a very difficult year for the midfielder absent through injury (302 days to be exact), Maniatis returned to action on 7 January 2016, in his first appearance in the 2015–16 season where he scored the last goal in a glorious 4–1 away win against Chania for the Greek Cup.

[17] After a frustrating six months period for the 2016–17 season, the experienced defensive midfielder and captain of Olympiakos is expected to leave the club in January, to continue his career elsewhere, as was not in the plans of Paulo Bento.

[18] On 3 February 2017, after a frustrating first half season, Olympiacos officially announced the mutual termination of experienced central midfielder and captain of the club contract.

Maniatis was not in Olympiakos' head coach Paulo Bento's plans for the 2016–2017 season and after failing to make a single appearance throughout the first half, his contract was mutually terminated in February 2017.

Shortly after he signed for the west Athenian club Atromitos on a free transfer for the remainder of the season, with an optional extension clause until June 2018.

[24] On 8 February 2019, after eight years away, Giannis Maniatis returned to Panionios, signing a contract with the Nea Smyrni club after being released from Alanyaspor over the winter transfer window.

[26] On 13 November 2019, after a rather mediocre for the player season, in addition to an episode with team manager Nikodimos Papavasiliou, resulted in mutual termination of his contract.

[27] After his impressive performances at Panionios, Maniatis was called up to the national team on 11 August 2010 by Fernando Santos to face Serbia where he made his debut coming on as a substitute in Greece's 0–1 away win.

[30] The Greece midfielder was so enraged after a training ground spat that he booked a himself on a flight back to Athens before being persuaded not to walk out on Fernando Santos's squad.