Lampard used the club's in-house television channel to deliver a withering assessment of the Blues' season, which sees them in real danger of failing to qualify for next season's Champions League.
Di Matteo admitted Chelsea - who are five points adrift of the Barclays Premier League top four - had endured a turbulent campaign.
"The numbers say that, so there is no point in denying it's been a difficult season,'' he said.
"But we have quality in this team and many good players. I still think this Chelsea team is a very good team.''
Di Matteo, whose side go into their quarter-final first leg in Portugal as England's sole remaining European representative, added: "Chelsea used to be in the top two or top three, at least. So it's been hard for us, domestically.
"But we're in the semi-finals of a domestic cup, and in the quarter-finals of the Champions League.''
Lampard also admitted Chelsea had allowed their intensity level to drop since their historic last-16 comeback to beat Napoli.
Di Matteo added: "I have faith in this team. They've shown they can play at high intensity at a high level, so I'm looking forward to my team doing that again tomorrow.
"I am positive about the team, but my players are positive too.
"We have great belief in our abilities. That's what we're going to try and express tomorrow.''
Benfica boss Jorge Jesus, meanwhile, told Didier Drogba his side would react to a perceived slight from the Chelsea striker by winning tomorrow's clash in Lisbon.
Benfica appear to have taken offence at a video on the London club's official website which some have claimed shows Drogba feigning fear at having drawn Jesus' men.
Chelsea defender David Luiz - the former Benfica player - suggested this evening that Drogba was actually reacting to the prospect of facing Barcelona in the semi-finals, insisting his team-mate had not been disrespecting Benfica.
Jesus accepted the forward was "probably just joking'', but added: "Drogba only knows what he's thinking.
"You have to remember that Benfica and Sporting Lisbon eliminated Manchester United and Manchester City, the two top teams in England, from Europe.
"Any responses we'll be giving to Drogba will be on the field and proving we are better than him.''
Luiz attempted to clear up the matter, which Chelsea themselves branded "a complete misunderstanding''.
Luiz said: "If people watch the video closely, it's not directed at Benfica.
"He was talking about Barcelona because Barcelona are a very, very big team.
"Chelsea know how Benfica reached this far. They're a strong team.
"Drogba's character wouldn't allow him to disrespect another team.''
Source:soccernet.espn.go.com
