Rangers manager Gio van Bronckhorst was left baffled after Dejan Stankovic appeared to snub a post-match handshake.
The Premiership champions took a thumping 3-0 win in the first leg of their Europa League last 16 tie against Red Star, putting one foot in the quarter-finals.
Captain James Tavernier opened the scoring from the spot, before Alfredo Morelos smashed home on a set piece.