Celtic lost the second leg 3-1 to Sion to lose by the same score on aggregate, leaving Scotland without a representative in either the Champions League or Europa League group stages.
"It seems to me that Sion have completely broken the tranfer ban imposed on them and are in direct violation of the rules and Celtic could come back in," UEFA president Michel Platini told reporters ahead of the draw for the group stage.
"But we must wait and see what the outcome is, it is a legal issue that has to be resolved. But to me the issue seems clear and Sion have broken the terms of the ban."
UEFA General Secretary Gianni Infantino added: "We expect to receive Celtic's appeal in what has been a long-running saga regarding Sion.
"The Disciplinary and Control body will decide on the issue before the group stages start, although Sion's name will be in the draw. Sion has the right to go to CAS (the Court of Arbitration for Sport), who will settle the matter."
Sion, who were banned from buying or selling players for two transfer windows, have already taken their case to FIFA and the Swiss Supreme Court but a provincial court ruled in their favour last week, prompting the current discrepancy.
Sion were banned after FIFA ruled the club was guilty of inducing Egypt goalkeeper Essam El-Hadary to break his contract with Cairo club Al Ahli in 2008.
Sion claim they have already served the ban but FIFA say it runs until the end of this year.
Sion had five players in their squad - Feindouno, Mario Mutsch, Gabri, Jose Goncalves and Billy Ketkeophomphone - who are, as far as the authorities are concerned, ineligible.
However, following the provinicial court's ruling, the squad was approved by the Swiss FA who, along with the club, have reportedly been warned by Infantino they could face more sanctions with Sion expelled from the competition.
