Lampard's first game in his second spell as Chelsea boss, albeit on an interim basis, was decided by Mattheus Nunes' fine first half strike.
The loss was Chelsea's eleventh in the Premier League this season, leaving them in 11th position and far away from the top four places.
Chelsea's run of games does not get any easier as they travel to Real Madrid for the first leg of the UEFA Champions League quarterfinal on Wednesday.
Lampard has highlighted areas for improvement after the defeat to Julen Lopetegui's side.
"We know we're not in the position we want to be and there is always a reason for it," Lampard said. "I was aware of that, and I did not expect to solve everything in one day.
"If you're analysing it, in a Premier League game you have to have more aggression in your game, more speed, more competitive duels that go your way, rather than the opposition.
"That is baseline stuff against a team fighting for their Premier League lives. It will always dictate the result of the game, regardless of the talent.
"There was a little bit of that today and the players need to be aware of that, because without that you can't win games and you can't be the team that Chelsea has been at different times, and we want to get back to."
He added: "I am here to help with that, and this is not stinging criticism, but what are the reasons we can be like that at times?
"I know there is a lot of talent there in the squad, I have seen that in the last couple of days, and I look forward to working more with it."
