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Chelsea suffered a surprise 3-2 defeat to West Ham, who moved three points clear of the relegation zone thanks to Andriy Yarmolenko's 89th-minute winner at the London Stadium.
Willian's brace was not enough for Chelsea to secure at least a point away from home – the result ending the club's six-game unbeaten streak in all competitions.
Asked if he was concerned that Chelsea had a mental issue following their inconsistent form in 2019-20, head coach Lampard told reporters: "It's something that has happened a few times this season.
"We've had opportunities to close gaps and jump over teams because of other results or whatever situation.
"But we haven't done it and that's a sign of where we are. We know we have a lot of hard work to do to get where we want to be and the reason the rest of us are chasing Liverpool and Man City is because of the consistency that develops over time. That's what we have to work towards.
"It's obviously frustrating but if we're in that position it's how you bounce back, how you move forward. We have Watford now at the weekend and games after that to try and carry on.
"We have to realise the position we are in in terms of fighting for the Champions League places and move forward."
While Chelsea – who will welcome relegation-threatened Watford to Stamford Bridge on Saturday – lost their 10th game of the campaign, Christian Pulisic was a shining light against West Ham midweek.
Pulisic earned the 42nd-minute penalty which Willian converted before winning the free-kick that was also put away by his Brazilian team-mate with 18 minutes remaining.
"A big positive," Lampard said. "He's playing really well, as we can all see. Him and Willian as well. Willian gets his two goals. But with Christian I think it's more like earlier in the season when he had a patch in the run up to Christmas where he was playing towards his level.
"Then he got injured and has been out for a long time. It's nice to see him back, fresh and hungry, and playing with the quality he has. He's a young player with great talent."