The assembly comprises a bearing unit (10) with outer (11) and inner (12) rings and an outer wheel (20) of plastic material which is overmolded onto the outer bearing ring and is adapted for being engaged by a driving belt. A sleeve unit (30) is integral with the inner bearing ring (12). The sleeve unit includes one or more rigid tubular spacers (31, 32) and a central, plastic tubular core (33). The spacers (31, 32) are axially aligned with the inner bearing ring (12) and abut against one or both of the opposite sides (14, 15) of the inner bearing ring (12). The central tubular core (33) is overmolded into and axially extending along radially inner cylindrical surfaces (16, 34, 35) of both the inner bearing ring (12) and tubular spacer(s) (31, 32).