avert joined the Premier League outfit in a reported £72million deal but he is yet to hit the ground running in England.
The 21-year-old failed to impress in his Premier League debut in a 3-1 win over Brighton at the AMEX Stadium last week.
He followed up with a similar indifferent display against champions Liverpool as Chelsea succumbed to a 2-0 defeat at the hands of the Reds over the weekend.
Lampard believes Havertz is not up to speed and will prove his worth very soon.
"Kai has played various positions in career, as number nine, number eight and a number 10," the former Derby County head coach said.
"He played lots off the right for Leverkusen and I have my own vision going forward.
"The key now is getting him fit.”
“Kai, with the situation we are in we aren't in a position where we can wait. We need to get him fit, and it'll become very clear why he's here."
Chelsea will be looking to atone for their weekend defeat against Jurgen Klopp’s men when they face Championship side Barnsley in the Carabao Cup on Wednesday and Lampard is happy to have an extra 90 minutes away from the Premier League to help nurture his men back to full fitness.
"We’re in the position that this game for me is of course about getting through to the next round, it always would be, but I have huge eyes on the fitness of the team because we’re not fit now, which means our pressing game and the speed of our game, it will be very difficult to attain the levels that we want," he added.
“So I’ll use this game primarily for the first-team squad to get there. With respect to Barnsley and respect for the competition, it’s a huge night for us for the fitness of our team.
"We’re in a very difficult position. I don’t want to compare us to all the other teams, but we had pretty much as short a break as you can have because of our Champions League game against Bayern Munich, which took us two weeks beyond the regular season, and then we came back and played when everyone else started, unlike a few other teams in the Premier League.
"We also made some new signings, we also had double figures of quarantines at the start of pre-season, so we’re clearly in pre-season mode and that’s why we mustn’t get too judgmental.”
“I’m not being too judgmental of the squad at this point because we are short on fitness for individuals in the team."
