Brendan Rodgers’ side will drop into the round of 32 stage if they avoid a four-goal defeat in their final Group B outing at Celtic Park.
But, while Dembele and Nir Bitton are fit to play, winger Patrick Roberts misses out with a long-term hamstring problem.
The former Fulham striker sat out two league matches with Motherwell after scoring against the same opponents in the League Cup final.
“We played very well in the first game over there and now we look to finish it off at home,” said Rodgers.
“They will come to attack and look for a victory but that’s also what we want. We want to give the supporters a Champions League win at home.
“That would take us to six points, which would show a great level of progress for us at this level.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.