Sri Lanka crashed to 136 all out after being put into bat first and the Kiwis chased down the target in 16.1 overs.
Martin Guptill (73) and Colin Munro (58) hammered chased down the target after Matt Henry and Lockie Ferguson claimed three wickets each to rout Sri Lanka.
But Karunaratne said all is not lost.
"I think we have another eight games to go. We need to (keep our) chin up and play very well," Karunaratne said in the post-match press conference.
"We need to see what we have. On the back end, we need to get some partnerships, (and in) the middle part. Those are the key areas. We're looking forward to doing it."
Sri Lanka received some booing by a group of Sri Lankan fans who were frustrated by the tame nature of their team's loss.
Karunaratne said losing the toss on a green Sophia Gardens wicket was not the ideal start, but gave no excuses for Sri Lanka's meek surrender.
"They (fans) were far from Cardiff and they spend their time and money to come here to see a good match," said Karunaratne.
"I think that's the main thing. If we get out for 130, you know, there is no match. We have to give them a good show at the end of the day."
Demanding his batsmen to learn from their mistakes, Karunaratne said: "If you take this wicket, you know, you need to have a target. It's not easy. It's not a 300 wicket, so you need to have another game plan.
"You can't go there and hit every ball, so you have to stay calm. I think in these conditions, 250 or 240 will be a good score.
"I think they bowled really well, so we have to give them credit."