A 2-1 victory at the KCOM Stadium on Saturday carried Chelsea through to the fifth round, but they were clinging to their lead late on against the Championship side.
Head coach Lampard knows where Chelsea are lacking, despite expressing enthusiasm for goalscorer Michy Batshuayi's performance against Hull.
Chelsea's transfer ban prevented Lampard making signings when he came into the club in the last close season, but they are free to bring in players now, and the countdown to the window closing at the end of January is on.
"My job, I suppose, is to work every day on the training ground," Lampard said on BT Sport. "The other part for the club is how do we move forward by bringing in players, and we lost ground last summer.
"We're trying to now, we need to now for the short term to push for this top four [in the Premier League]. I have a strong feeling about where we can improve personnel-wise to take us forward."
"Recruitment when you've missed a [transfer] window is of course important and in the short term now it feels very important," Lampard said.
"That's something we're working on and we have to get it right."
Fikayo Tomori headed Chelsea's second goal just after the hour mark, but a deflected free-kick gave Hull a late lifeline, which they almost grabbed.
Speaking in his post-match news conference, Lampard admitted Chelsea's failure to build on their first-half dominance was disappointing.
"You don't expect it to be easy coming here, but we probably made it harder on ourselves by not capitalising," Lampard said.
"Hull probably weren't at it in the first half, I think we had chances to score. Then with passages we got slightly sloppy. At half-time I wasn't delighted and then in the second half Hull gave us a real game of it.
"If you don't take those chances when you can then teams stay in it for 90 minutes, like they did.
"I don't know how many shots we had. Recently we have been having a lot of good chances and not taking them."
Chelsea would love to have a driving force in the Lampard mould, given he became the club's record scorer in his playing days.
The loss of Abraham to an ankle problem has left the team short of their most clinical finisher, and Lampard said he was still unsure of the exact nature of the injury sustained by the striker in the midweek draw with Arsenal.
"He has been getting lightly into the water and all these things, but I said [on Friday] that there's no fracture, but other than that there is still some swelling so I don't know," Lampard said.