Frank Lampard was delighted with the way Chelsea rallied from an "awful" first-half display to beat Arsenal at Emirates Stadium on Sunday.
The Blues' inconsistent form continued with a 2-1 victory after late goals from Jorginho and Tammy Abraham, having looked set to lose for the second match running following Pierre-Emerick Aubameyang's opener.
As in a Boxing Day defeat to Southampton, Chelsea were unrecognisable from the team that dominated Tottenham in an away win a week ago.
But Lampard saw his side's "lethargy" give way to a spirited second-half display, with the coach refusing to take credit for the triumph despite a tactical tweak, ditching the 3-4-3 system that proved Spurs' undoing.
"We were so awful for 30 minutes; slow, lethargic, nervous, the opposite of against Tottenham," he told BBC Sport. "We gave Arsenal everything they wanted.
"We made the change early, felt it had to be done. In the second half, we dominated.
"After 10 minutes or so, it was clear that Arsenal had seen us play against Tottenham and made us into a back five instead of a back three.
"They were on top and there was a lack of balance to our team. The sub gave us another man in midfield and gave us more urgency.
"From my point of view, you can accept a missed pass but you can't accept lethargy in a London derby. The players were told that [at half-time] and they delivered.
"The second half was nothing to do with tactics, it was all to do with spirit and desire."
The dramatic victory still was not enough to distract from Chelsea's failings against Southampton at Stamford Bridge for Lampard, though.
"I still haven't bounced back from Boxing Day mentally," he said. "In our next home games, we'll do everything we can to turn the corner.
"We're still in fourth and want more, and the home games have been different so we have to focus on them when they come around."