The Polish side wrapped up a one-sided Champions League qualifying round victory over Celtic on Wednesday, a 2-0 win at Murrayfield in Edinburgh earning them a 6-1 triumph on aggregate.
But Warsaw have confirmed that they have now been asked to supply information to European football’s governing body regarding defender Bereszynski, who came on as a sub with three minutes left.
A statement on Legia's official website read on Thursday: "The club announces that, in connection with the Celtic FC-Legia Warsaw match in the second leg of the third qualifying round of the Champions League, UEFA opened an investigation into the participation of Bartosz Bereszynski.
"The club is preparing the relevant information and explanations, which will be sent to UEFA today.
"The result of the investigation will be announced immediately after its completion."
Legia are through to the Champions League play-offs but could be thrown out of the competition if they are found guilty, with Celtic taking their place.
The investigation concerns Bereszynski’s sending-off for violent conduct against Apollon in the final match of Legia's Europa League campaign last season.
He missed both ties against St Patrick's in the second qualifying round and then sat out the first-leg against Celtic.
However, the investigation is apparently focussing on whether the player was registered in Legia's squad for the games against the Irish side in order to allow a three-match suspension to take effect.
Bereszynski only came on in the 86th minute of Legia's 2-0 win at Murrayfield when the game was well and truly over but the context and impact of his appearance might not have any influence on the outcome if rules were breached.
UEFA's disciplinary regulations decree that a match may be declared a forfeit if an ineligible player participates, "as long as the opposing team files a protest."
The team forfeiting the match is deemed to have lost 3-0 which would put Celtic through on the away goals rule.