Barca was comprehensively beaten 3-0 by Bayern Munich on matchday one and this was every bit as humiliating as the Catalans lost back-to-back group games in a single season for the first time since 2000-2001.
A nightmare start quickly gave them an uphill battle as Darwin Nunez put Benfica ahead in the third minute, and although Barca was soon dominating, they were extremely wasteful in front of goal.
Marc-Andre ter Stegen got away with a dreadful error early in the second half but Benfica held firm at the back and eventually seized its opportunity to finish Barca off through Rafa Silva and a late Nunez penalty before Eric Garcia saw red to cap a dreadful night for Koeman.
Garcia offered little resistance as Nunez earned Benfica an early lead, the Uruguayan producing a couple of stepovers before finding the bottom-left corner.
Barca quickly began to dominate in response, but a fantastic Lucas Verissimo block prevented Luuk de Jong from finding an empty net.
Frenkie de Jong was similarly wasteful when prodding a low cross wide from close range, and Pedri – a surprise starter so soon after injury – shot agonisingly past the left-hand post a little later.
Ter Stegen summed up Barca's poor decision-making early in the second half, bizarrely opting to rush out in an attempt to cut out a throughball and was beaten to it, but Nunez could only hit the post.
Luuk de Jong then somehow hit the upright from point-blank range at the other end, though an offside call against Ronald Araujo in the lead up spared his blushes somewhat.
Either way, Barca was made to rue its profligacy.
Silva slammed home from 10 yards after Ter Stegen parried a shot right to him and Nunez made it 3-0 with a cool penalty after Sergino Dest handled, with Garcia's late red card for a cynical tug compounding Barca's embarrassment.