Pierre-Emerick Aubameyang scored twice as Barcelona eased to a 3-1 win over 10-man Celta Vigo in a game overshadowed by Ronald Araujo leaving the field in an ambulance.
Araujo clashed heads with Gavi shortly after the hour mark at Camp Nou and, after initially appearing to run off the injury, collapsed to the floor before being attended to by medics.
That incident came shortly after Iago Aspas had struck to give Celta some hope following Memphis Depay's opener and a couple of goals from Aubameyang either side of half-time.
But a red card for former Barca loanee Jeison Murillo ultimately left the visitors, who recovered from three down to draw 3-3 in the reverse fixture, with too much to do in Tuesday's LaLiga contest.
Dembele showed superb trickery to take the ball past two opponents and pull it back for Depay to sweep past Matias Dituro, who got a hand to the shot but could not keep it out.
Marc-Andre ter Stegen, returning from illness that ruled him out last weekend, kept out Thiago Galhardo at full stretch and denied the same player from a header soon after.
Fourth goal ruled out
Barca doubled their advantage before the interval through Aubameyang, who tucked into the bottom-right corner after Nestor Araujo failed to deal with Depay's cross into the box.
Aubameyang took advantage of some more good play from Dembele three minutes after the interval by clinically sweeping home Barca's third.
A poor pass from Ter Stegen to Araujo allowed Galhardo to pounce and play in Aspas, but Celta's momentum was halted when last man Murillo fouled Depay and was sent off.
Barcelona saw Araujo leave the field with a head injury soon after and had a fourth goal for substitute Riqui Puig ruled out for offside, but they had no trouble seeing out the win, despite 11 minutes being added on.
