Mazzù has built a reputation for improving the status of smaller clubs, earning a number of promotions with regional sides, before helping Charleroi qualify for the Belgian Pro League title play-offs three times.

He is also known for spotting and nurturing young talented players, such as Victor Osimhen, Paul Onuachu, Kaoru Mitoma, Deniz Undav and Dante Vanzeir.

[13][14] In his final season at Charleroi, Mazzù signed Victor Osimhen on loan from Wolfsburg, at that stage having not scored in European football, with a return of no goals from 16 appearances for the Bundesliga side.

[16] However, a poor start to their title defence allied with Genk's early UEFA Champions League exit in 2019-20, finishing fourth in a group also containing Liverpool, Napoli and a Salzburg side containing Erling Haaland and Takumi Minamino, saw Mazzù dismissed.

[19][20] A sensational return to top-flight football after 48 years away saw a 3-1 opening-day win away to Brussels' traditional giants Anderlecht,[21] and a week later, Union hosted reigning champions Club Brugge on 1 August 2021 in their first home game in the top division since 1973, being edged out 1-0 through a late Eduard Sobol winner.

In a difficult January run against all of the previous season's top four, Union beat Anderlecht, Genk and Royal Antwerp, drawing 0-0 away to Club Brugge.

Club Brugge won the midweek return 1-0 at the Jan Breydel through a rebounded own goal from goalkeeper Anthony Moris to take a three-point lead with two games to go.

[23][24] Club Brugge clinched the league crown in their next (and penultimate) game against Royal Antwerp,[25][26] despite Union beating Anderlecht for the fourth time in the season 2-0 at Lotto Park.

The Unionistes' impressive run to becoming Brussels' top club attracted interest across the capital, and within days of being named Pro League Coach of the Year for 2022, Mazzu was photographed speaking in a restaurant to representatives of Anderlecht, who had just decided to dismiss Vincent Kompany.

[41] But among major in-house politicking and a poor start to the season, Mazzù was sacked on 24 October 2022 in the aftermath of an abandoned match away to Standard Liege, due to Anderlecht fan unrest.

The following 2023-24 season was a struggle, with Charleroi losing Belgium international Joris Kayembe to Genk, alongside the departures of attackers Amirhossein Hosseinzadeh, Jackson Tchatchoua and Ali Gholizadeh.