"We have some experienced players in the middle with Kusal Mendis and Angelo Mathews," batting coach Lewis said on Monday.
"They obviously haven't quite fired yet but Afghanistan let us get away with that. I'm confident players of that quality will definitely come to the party soon.
"With Angelo you have to have faith in a guy who has done as much as he has over a long period of time and he tells me he is in a good place and good headspace."
"I have no reason to doubt that the runs will come from him," said Lewis of the 32-year-old all-rounder.
While Mendis has six centuries in 38 Tests, the 24-year-old, in contrast, has just one ODI hundred from 65 matches.
Lewis, when asked to explain the difference in records, said: "Mendis could do with a little bit of a confidence boost. I think for him it's a case of just getting through those first 20 balls.
"In the build-up to the World Cup, he played well against Scotland and the two warm-up games he looked in good form even though he only got 20s and 30s, which was a downer.
"Those would have been a good opportunity to get a big score, but there was nothing about his form in either of those two games to suggest there's anything to worry about.
"He's had two low scores and of course it would be good if he had more runs behind him, but I think he's probably only 20 balls from being really away.”
Lewis, former coach of English county side Durham, confirmed Sri Lanka would be without their paceman Nuwan Pradeep, who took four for 31 against Afghanistan.
He suffered a finger injury during a practice session on Sunday. Suranga Lakmal is set to be brought in as a like-for-like replacement.