The Stoke City loanee played 90 minutes of Celtic's 1-1 draw at Hibernian on Saturday.
It was his first game back after missing the drubbing of Partick Thistle through injury in midweek.
But according to The Scottish Sun, Bauer has sustained a 'fresh ankle knock' and Lennon reportedly revealed after Saturday's stalemate that he was unsure whether the right-back would make Thursday.
The 27-year-old has looked a good signing since Lennon brought him on loan from the English side, who paid £5 million for him in 2018 [The Daily Mail].
And with Jozo Simunovic already ruled out for four months, the Celtic boss will be hoping and praying that there isn't anything seriously wrong with the Swiss defender.
Lennon has Hatem Elhamed to deputise against Cluj, but Bauer has arguably looked the stronger of both right-backs since arriving.
The Bhoys' first Europa League game this season ended in a 1-1 draw away at Rennes.
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