Kasey Bos, a 22-year-old Australian-Dutch left-back, will play for Excelsior on loan for the next season, a move announced on the Rotterdam club's official channels. He is being loaned from FSV Mainz, where he moved last summer from Melbourne. Bos has not yet made his official debut for the Mainz first team, in part because of a shoulder injury. During the 2025/2026 season, he made 14 appearances and provided 4 assists for the club's reserve team in the Regionalliga Südwest.
Mark Ruijl, Excelsior's technical director, revealed that the club was looking for a new left-back following the departure of Arthur Zagré and that they were familiar with Bos from his time in Australia. Ruijl described Bos as having immense running ability, intensity, and dynamism, stating these are qualities that fit the club's style of play. Ruijl added that Bos views Excelsior as the ideal environment to regain his rhythm and form.
Bos stated that he has a four-year contract at Mainz and feels the move is a good step for his career and an opportunity to show what he can do. His current market value is estimated at €450k, according to Transfermarkt.
