Demebele, who missed two chances to score during Wednesday's 3-0 win over Liverpool in the first leg, was one of few regulars to face Celta as coach Ernesto Valverde made 11 changes to the side that clinched the La Liga title last week.
However, the France forward, who has been plagued with muscle injuries since completing a 105-million-euro (89.3 million pounds) move from Borussia Dortmund in 2017, had to be taken off in the sixth minute.
Barca said in a statement the 21-year-old would undergo further tests on his right hamstring on Sunday.
"We'll wait and see what the tests say tomorrow but obviously he's a very important player for us and it's a big setback to lose him for the next few games," said Barca coach Valverde.
Dembele has scored 14 goals this season for Barca in all competitions but they will still be confident of seeing out the tie with Liverpool without him after two goals from Lionel Messi and one from Luis Suarez gave the Catalans a commanding first-leg win.
Liverpool's chances of overhauling the three-goal deficit looked even bleaker on Saturday after Mohamed Salah was carried off during their 3-2 Premier League win at Newcastle United. The Egyptian suffered a head injury following a collision with Newcastle goalkeeper Martin Dubravka in the second half.
But coach Juergen Klopp said the striker was not seriously injured.
"What I heard, he got the hip of the goalie on his head. He was then on the ground and the doctor had to make a decision 'on the pitch or off the pitch', and the decision was off the pitch. We accept that of course," Klopp said.
"When he came in he was sitting in the dressing room watching the game on television. So he was then fine, but of course we have to wait."
There has been an increased spotlight on head injuries in football after Tottenham Hotspur defender Jan Vertonghen continued playing in his side's Champions League semi-final with Ajax Amsterdam last Tuesday after taking a blow to the head, before eventually being substituted.
(Reporting by Richard Martin, editing by Pritha Sarkar)
