Legia Warsaw have admitted they are under investigation for fielding an ineligible player during their Champions League win over Celtic.
Bartosz Bereszynski played in the final two minutes of the Polish champions' 2-0 victory at Murrayfield on Wednesday, a result that earned them a 6-1 win.
But Uefa is investigating whether the full-back should have been suspended.
"Uefa has opened an investigation on the participation by Bartosz Bereszynski," read a Legia statement.
"The club is preparing relevant information and explanations, which will be sent to Uefa today.
"The outcome of this case will be informed immediately after its completion."
Bereszynski was sent off in Legia's final Europa League tie last season against Apollon Limassol and received a three-match ban.
Uefa will assess the case at a meeting on Friday before the draws for the next rounds of the Champions League and Europa League later that day.
Celtic have previously been given a reprieve in continental competition in similar circumstances.
Neil Lennon's side were beaten 3-1 on aggregate by Sion in the Europa League three years ago, but the Swiss side were subsequently ejected from the competition after they were found to have fielded ineligible players.
Sion had signed five players while still under a Fifa transfer ban for breaching rules over the signing of Egypt goalkeeper Essam El-Hadary three years earlier.
However, in 2010, Uefa fined Debrecen £15,000 but rejected a protest from opponents Litex Lovech, who wanted the Hungarian side kicked out of the tournament for fielding an ineligible player.
Debrecen argued that they fielded Peter Mate in good faith as a substitute in Bulgaria when they were leading 4-1 on aggregate.
Uefa ruled that Debrecen had "no interest in fielding this player for the three last minutes of additional time, when the score was so clearly in its favour".
Should Celtic be reinstated to the Champions League, they will be seeded, with their potential opponents being Ludogorets Razgrad of Bulgaria, Malmo of Sweden, Aalborg of Denmark, Maribor of Slovenia or Slovan Bratislava of Slovakia.