The Polish champions progressed comfortably to today's draw for the play-off round by beating Ronny Deila's side 4-1 in Warsaw and then 2-0 at Murrayfield in Wednesday night's return leg. The 6-1 aggregate loss equalled the worst two-legged scoreline in Celtic's history.
That left the Scottish champions awaiting today's Europa League play-off round draw but they could yet find themselves back in the Champions League after Legia released a statement on their website revealing that UEFA were looking into the appearance of Bartosz Bereszynski in the second leg in Edinburgh.
The 22-year-old defender was sent off last season against Apollon Limassol in Legia's final Europa League group game and it was thought he had served his three-game suspension when he didn't feature in either leg of the Poles' second qualifying round victory over St Patrick's Athletic of the Republic of Ireland, or in the first leg against Celtic in Warsaw. He then appeared as a substitute for the closing four minutes of the return match at Murrayfield.
It is understood, however, that the club did not register him with UEFA for the match against St Pat's and, if correct, it would mean his ban had not been fully served and he was ineligible to face Celtic.
The statement on the Legia website said: "The club announces that in relation to the second leg of the third qualifying round of the Champions League between Celtic FC and Legia Warsaw, UEFA has opened an investigation relating to the participation of Bartosz Bereszynski.
"The club has prepared relevant information and an explanation which will be sent to UEFA today. The result of the investigation will be made known immediately after its completion."
If UEFA found that Bereszynski was ineligible to play at Murrayfield, Legia would forfeit the second leg and Celtic would instead be awarded a 3-0 victory. That would be enough for them to progress to the Champions League play-off round courtesy of the away goals rule.
Celtic benefited from a similar scenario in 2011 when the Swiss side, Sion, were found to have fielded ineligible players in their 3-1 Europa League win over Neil Lennon's side. Sion were eliminated from the competition and Celtic took their place in the group stage.