Darwin Nunez scored twice, the second a penalty, and Rafa Silva was also on target as Barca suffered back-to-back defeats in the Champions League group stage for the first time since 2000. Eric Garcia was also sent off late on after receiving a second yellow card to compound the misery on a Barca side who lost their first game of the competition 3-0 to Bayern Munich at Camp Nou.
Barca still have four games to get their act together. They're not out just yet. Their next two games will be key, with both coming against Dynamo Kiev. They then host Benfica. They're six points behind leaders Bayern and four off Benfica.
It's been 21 years since Barca last failed to get out of the group stage. After this result, that looks like a very real possibility this season. That is unacceptable for this club.
The defence was shown up against Benfica. Gerard Pique was taken off after half an hour before he was sent off and Garcia, who was beaten easily for Nunez's first goal, was later dismissed for a second booking. Sergino Dest, meanwhile, could have avoided the handball that gave away the penalty, while Marc-Andre ter Stegen was lucky Nunez didn't score in an empty net after a rush of blood to the head left the Germany goalkeeper stranded 40 yards out of his goal.
Manager rating out of 10
2 -- Koeman meddled with the team that had beaten Levante so convincingly at the weekend and then proceeded to make a series of puzzling changes. While Gavi's early introduction for Pique made sense, dropping Frenkie de Jong, Barca's most dangerous player in the final third until that point, back into defence did not. In the second half, Barca lost their way as the game descended into chaos.
Player ratings (1-10; 10 = best, players introduced after 70 minutes get no rating)
GK Marc-Andre ter Stegen, 5 -- Beaten at his near post for the first goal and was lucky not to concede a second when caught in no man's land. Parried the ball in the build-up to the second goal, but the finish was good.
DF Eric Garcia, 3 -- Hasn't looked up to scratch since signing from Manchester City. At fault for the first goal and later sent off for the second time this season.
DF Gerard Pique, 4 -- Taken off after 30 minutes. Had already been booked and was lucky not to get a second caution.
DF Ronald Araujo, 6 -- Made a couple of good challenges and fought hard, but was only as good as his defensive colleagues in the end.
MF Sergi Roberto, 4 -- Returned as a right wing-back, despite Koeman saying after the Bayern hammering "it's not his position." That was obvious again. The player's confidence is low.
MF Sergio Busquets, 5 -- Couldn't handle the physical battle in midfield and was taken off in the second half.
MF Pedri, 6 -- Was good early on but tired on his first game back from injury. Incisive passing created chances and he had a couple of shots of his own. One was blocked by Luuk de Jong, his own teammate, and another fizzed wide.
MF Frenkie de Jong, 6 -- Really good in the final third for half an hour, setting up a chance for his namesake Luuk and then another for Memphis Depay. Was strange, therefore, to see him dropped back.
MF Sergino Dest, 4 -- Didn't have as much joy running at defenders as he did against Levante and then gave away a late penalty.
FW Memphis Depay, 5 -- Missed the best chance that fell to him in the first half, his skewed shot deflected wide.
FW Luuk de Jong, 3 -- Missed a great chance to equalise in the first half and then blocked Pedri's follow up. Missed another sitter after the break, although the flag would have gone up if he had scored.
MF Gavi, 5 -- Did so well on the right against Levante so it was a surprise to see him on the bench.
FW Ansu Fati, N/R -- Unable to have the impact he had at the weekend but the minutes will help him as he comes back from 10 months out.
MF Philippe Coutinho, N/R -- Introduced with Barca in a mess and he could not change that.
MF Nico Gonzalez, N/R -- Like Coutinho, the youngster came on seconds before the second Benfica goal and the game ended there.
DF Oscar Mingueza, N/R -- Came on for two minutes after the third goal.