Former Anderlecht general manager Herman Van Holsbeeck was held on Thursday as part of a corruption probe linked to the transfer of Serbian striker Aleksandar Mitrovic, a source close to the investigation told AFP.
The detention came a day after the arrest in Monaco of agent Christophe Henrotay -- whose clients include Belgium and Real Madrid goalkeeper Thibaut Courtois -- as part of the same probe.
The investigation centres on alleged illegal payments made as part of Mitrovic's moves from Partizan Belgrade to Anderlecht in 2013 and then from the Belgian side to Newcastle United, the source said.
The 24-year-old Serb, who has enjoyed a bright start to the season with five goals in six games for Championship side Fulham, moved from Anderlecht to St James' Park for 18.5 million euros (13 million pounds) in 2015.
Belgian daily Le Soir reported that Henrotay is suspected of making illegal payments to Anderlecht bosses including Van Holsbeeck at the time of Mitrovic's move from Partizan for around five million euros -- then a record for the 34-time Belgian champions.
The 64-year-old Van Holsbeeck was a top figure at Anderlecht for around 15 years before being removed when new management took over in 2018.
A spokesman for Belgian prosecutors confirmed an arrest in the investigation, without giving names.
Police launched seven raids on Tuesday and Wednesday in Belgium, Monaco and London, making two arrests including Henrotay in the wealthy Mediterranean principality.
Eric Van Duyse, spokesman for the Belgian Federal Prosecutor's Office, told AFP that two men were being held after being indicted for "money laundering, forgery and the use of forgeries, private corruption and criminal conspiracy".
The inquiry is separate from Belgium's so-called "footballgate" scandal, which has resulted in 20 suspects being charged since October 2018, including agents, referees and club officials, in a vast investigation into fraud and match-fixing.