A UEFA investigation began following Legia's 6-1 aggregate win over Ronny Deila's side after the introduction of Bartosz Bereszynski, who should have been serving a suspension, in the 86th minute of the second leg on Wednesday.
Legia were already 2-0 ahead in the second leg, and five goals clear of their hosts, at Murrayfield when the attacker replaced Michel Zyro.
And Europe's governing body confirmed on Friday that Legia will now compete in the UEFA Europa League play-offs, with Celtic taking their place in the Champions League play-offs.
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