Ronald Koeman said there was "no point" comparing his Barcelona team to those from the past after defeat by Benfica
Barca - also beaten 3-0 by Bayern Munich earlier this month - have lost their opening two matches in the competition for the first time, and sit bottom of Group E.
Benfica got off to an energetic start and opened the scoring inside three minutes, when Darwin Nunez cut inside and finished low into the near corner.
Rafa Silva then clipped in a second after the break and Nunez netted a penalty in a comfortable victory for the home side.
"I'm not going to argue about the level of my team," said Koeman, part of the Barca side that won the European Cup in 1992.
"There's no point in comparing this with Barcelona teams of years gone by. That's as clear as water. I can only give you my opinion of my work at the club - I feel backed by my players and their attitude."
As well as his two goals, Nunez also hit the post from out wide after rounding stranded Barcelona goalkeeper Marc-Andre ter Stegen, who made a rash decision to charge out of his area.
Luuk de Jong missed a glorious chance to level for the visitors early in the first half when presented with a seemingly open goal.
But Barca only registered one effort on target and had Eric Garcia sent off for a second booking with three minutes remaining.
"Benfica were strong physically and they're quick. I think that for the first and second goals we just had to defend much better," added Koeman.
"We had really good scoring chances and that's how you change the direction of a match. The fact that Benfica put away the large majority of the few chances they actually created then that's the big difference between the two teams."
Unwanted history for Koeman's Barca
Koeman finds himself in charge of one of Europe's most successful clubs during one of their toughest spells in recent history.
The club's financial struggles, emphasised by the loss of Lionel Messi to Paris St-Germain, mean this side is a long way from the one that last lifted Europe's most-sought after club prize in 2015.
The five-time winners face the prospect of not featuring in the Champions League knockout stage for the first time since the 2003-04 season, when they did not qualify for competition.
The last time they failed to progress from the first group stage when competing was in 2000-01, which is also when they last suffered successive group defeats, to Besiktas and AC Milan.
Defeats by Bayern and Benfica in Group E this year mean Barca have lost their opening two matches in a European campaign for the first time since the 1972-73 Uefa Cup, when they were beaten in both legs by Porto.
They have also conceded six goals without scoring in the process and only one team has ever progressed with a record as bad - that was Lyon, who reached the last 16 in 2007-08.
Koeman has overseen eight group games as Barcelona boss but only Louis van Gaal, with five defeats in 32 games, has lost more than the Dutchman's three group-stage fixtures.
In fact, Barcelona have lost four of their past five Champions League matches, drawing the other, which is as many as they lost in their previous 39.
They next host Dynamo Kyiv, who sit one point above them after an opening draw with Benfica, with Koeman knowing anything but a win is likely to spell the end of his side's European hopes and potentially his time in charge.