Havertz struggled for fitness and form after contracting coronavirus in November.
Lampard said: “If people aren't taking the effects long Covid is having on players seriously then they will be very quickly.
“We are, certainly, because every case is different. I was listening to the radio on the way to work on Friday and they were talking about the vaccine.
“Now the Government are saying it's three months afterwards for the second vaccine but GPs are saying it maybe should be three weeks. Nobody knows, nobody knows what's going on.
“But what we do know is that long Covid is a serious thing.
“Some people bounce through it – Marcos Alonso came back from two weeks off and didn't feel anything. He came straight back into training.
“Other people are having fallout from it for months. So, everybody, we have to address it seriously.
“It's not an excuse, it's not a reason, it's a fact that players are having to get over this in different ways and some of them are finding it tough.
Havertz struggled badly with it, which is why Lampard is keen for Chelsea supporters to show patience with their £72million summer signing.
He added: “Just as Kai was just coming to terms with the Premier League – and he had some really good performances with us – he got Covid and he had Covid quite severely as people are really starting to open up about it now.
“I have read a few things recently about how much people can suffer with it. Some people are asymptomatic.
“He definitely has had a fallout with that physically and I speak with him a lot and we know that.
“It's something we are trying to help him deal with. So, yes, the expectation around him, you have to put context into the story.
“Kai's talent is undoubted. I see that every day. Giving him the time to adapt to the Premier League and to our team is crucial.
“I have full belief in him. We need to give him that time particularly because of the Covid situation, that is a problem."
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