Celtic are still not 100% certain of progressing in the Champions League by default. Legia Warsaw have submitted their appeal to the Court of Arbitration for Sport (Cas) after losing their Champions League place to Celtic following registration failures.
The Polish club fielded a player they later found out was suspended in the third qualifying round against Celtic.
Uefa subsequently punished Legia by awarding a 3-0 second-leg win to Celtic, which took the Scottish side through to the play-off round.
The governing body rejected an appeal, prompting Legia to appeal to Cas.
And Cas say it has "immediately initiated an arbitration procedure and the respondents Uefa and Celtic FC have been invited to file written observations".
"A decision on request for provisional measures is likely to be issued on Monday 18 August around 18:00 (Swiss time)," secretary general Matthieu Reeb added.
"Legia requests urgent provisional measures from Cas allowing the Polish club to participate in the play-offs of the Champions League (20 & 26 August 2014).
"More generally, the club requests that the Uefa decision be set aside and that all sanctions against the club and the player [Bartosz] Bereszynski be annulled."
Legia Warsaw manager Henning Berg Henning Berg guided Legia to two wins over Celtic but, as things stand, his side is in the Europa League
Following Celtic's reinstatement in the competition, the Scottish champions were drawn to face Slovenian side NK Maribor in the play-off round, with those ties scheduled for 20 and 26 August.
In the League Celtic have won back-to-back games to go top of the table.