The confusion arose after right-back Bartosz Bereszynski came on with just minutes remaining in the second leg. Bereszynski was sent off last season, but was not registered against St Patrick’s – Legia’s second qualifying round opponent – to serve his suspension.
Celtic crashed out in the third qualifying round with a limp 6-1 aggregate defeat.
A statement on Legia's official website read: "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 Bereszynskiego.
"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."
- Sports & Recreation