The German coach is set for a busy off-season rebuilding his struggling squad after the release of nearly half of his playing roster.
Joining Riera, Baumjohann and Bonevacia departing the Wanderers are Josh Risdon, Raul Llorente, Jaushua Sotirio, Abraham Majok, Marc Tokich, Rashid Mahazi, John Roberts and Giancarlo Gallifuoco.
The mass exodus at the club is the second in as many campaigns as the Rooty Hill-based outfit look to return to the A-League finals for the first time since 2016-17 .
Former Bayern Munich and Liverpool defender Babbel hasn't wasted any time rebuilding for next season with the Wanderers already announcing the signature of veteran fullback Daniel Georgievski.
Western Sydney will be expected to bring in a number of new overseas signings during the off-season with Bonevacia, Reira, Llorente and Baumjohann's departure opening up four of the club's five foreign player slots.
Baumjohann, who joined Western Sydney as a marquee signing in August, provided flashes of brilliance this season in 19 appearances for the Wanderers but was plagued by injury and inconsistency.
Spanish striker Riera, who was once a teammate of Lionel Messi at Barcelona's famed La Masia youth academy, started his Wanderers career with a splash in 2017 but never reached his full potential in two seasons with the club.
Socceroos Defender Risdon leaves Western Sydney after agreeing a deal to join A-League newcomers Western Melbourne next season.
Babbel's cull also includes a number of academy graduates including Tokich and Central Coast Mariners signing Majok.
