Supporters of the Polish outfit unfurled a large banner during the play-off second leg, depicting a pig wearing a money-themed suit in front of the UEFA logo, along with the slogan 'Because football doesn't matter. Money does.'"
The incident followed Legia's elimination from the UEFA Champions League for fielding an ineligible player in their third qualifying round second-leg clash with Celtic.
Legia, who had won the tie 6-1 over two legs, had an initial appeal rejected by UEFA before the Court of Arbitration for Sport sided with Europe's football governing body.
UEFA appear to have taken a dim view of fans' reactions, and could also take action against the club for "insufficient organisation".
A statement released on Monday read: "Disciplinary proceedings have been opened against Legia Warszawa for illicit banner (art. 16 (2) (e) of the UEFA Disciplinary Regulations), setting off of fireworks (art. 16 (2) (c) DR) and insufficient organisation (art 16 (1) DR) at their [UEFA Europa League play-off second leg played on 28 August in Poland].
"The case will be dealt by the UEFA Control, Ethics and Disciplinary Body on 4 September."