Roussey, who only took over from Bernard Challandes 14 months ago, stepped down a day after a 3-0 loss to Basel.
Sion have endured a nightmare of a season, not least becoming embroiled in legal conflicts which pitted them - unsuccessfully - against European governing body UEFA, who excluded them from the Europa League for violating FIFA transfer regulations.
The Lausanne-based Court of Arbitration for Sport (CAS) backed UEFA in the contretemps while a Swiss civil case brought by the club has been shelved by Swiss prosecutors.
The row erupted after the club, on whom FIFA imposed a transfer embargo over the signing of Egyptian goalkeeper Essam El Hadary, proceeded to sign six players in the summer.
The club then fielded five of the six during a Europa League play-off tie against Celtic and were kicked out of the competition.
Sion in addition were docked 36 points as a punishment.
That sanction would have meant relegation but Neuchatel Xamax were expelled from the top flight after 18 games for going bankrupt.