Despite dominating the game, the visitors only secured all three points with a superb solo effort from Van Dijk after just five minutes at McDiarmid Park.
"If we'd dropped points we probably would not have been talking about Virgil's goal," said Lennon.
"So I'm glad we are because it was an unbelievable piece of individual brilliance. He certainly lit up the game.
"I'm not expecting him to do that all the time but he's an exceptional player, a great defender and he's certainly made us better.
"He's added the physical presence and height we've been missing. He's been a good piece of business for us.
"It was closer than it should have been after the first half. I thought we controlled the game on a difficult pitch. But we have to put teams away.
"At 1-0 you give teams encouragement. But we've never had it easy here. St Johnstone came out for 15-20 minutes in the second half and were full of running and desire to get back into the game.
"But we saw it off and in the last 20 minutes we were okay again."
Saints manager Tommy Wright reckons his team should have been awarded a penalty when striker Stevie May appeared to run into Van Dijk as the Celtic defender turned to chase a deep cross.
"I thought it was a penalty at the time and I've seen it since," Wright said. "Nothing has changed my mind. Anywhere else on the pitch it's a free-kick so it should have been a penalty.
"I think we deserved to take something. Celtic created more chances but we contributed to a good game in the second half.
"In games like this you need a bit of luck and a decision here and there to go for you. We didn't get that today. But overall I'm pleased. We created a few chances against the best team in the country."