The club have confirmed there is an investigation into whether Bartosz Bereszynski - a late substitute in Wednesday's win over Celtic - should have been suspended
The Polish side emerged as 6-1 aggregate victors from their two-legged third qualifying round clash but face scrutiny for the involvement of attacker Bartosz Bereszynski.
The 22-year-old came on as an 86th-minute substitute in Wednesday's second leg, at which point Celtic required six goals to reach the play-off round, but was sent off in Legia's final Europa League tie last season against Apollon Limassol and received a three-match ban.
Uefa are now set to determine whether the player should have been suspended for Wednesday's game, with Legia announcing the news in a statement on their website on Thursday.
"The club announces that, in connection with the Champions League third qualifying round second-leg match Celtic FC - Legia Warsaw, Uefa has opened an investigation into the participation of Bartosz Bereszynski," it read.
"The club has prepared relevant information and explanations which will be sent to Uefa today. The result of the investigation will be announced immediately after its completion."