Lewis was the leading run-scorer in the side's series against Ireland last month, with 208 runs in three innings, but both he and Hetmyer failed to "attain the new minimum standard fitness requirements in recent fitness assessments," according to a Cricket West Indies (CWI) release.
Darren Bravo, the experienced top-order batsman, has won a recall following an impressive domestic season in which he made 481 runs at an average of 96.20 in the Super50 Cup, while allrounder Rovman Powell also makes the cut following an impressive run of his own: in the same competition, he scored 412 runs at an imperious strike rate of 160.93 for Jamaica.
Fabian Allen, who missed the India series with a knee injury, also finds a place in the squad, which will depart for a pre-series camp in Colombo on Saturday.
"Evin Lewis and Shimron Hetmyer missed out due to the fact that they came up short in the fitness test," said Roger Harper, CWI's lead selector. "They will be missed.
"Lewis was the team's best batsman in the recent series against Ireland where he batted with great composure and proved the bedrock for the team's series win. Hetmyer appeared to be getting his act together and was an integral part of the team's batting group.
"I look forward to the team building on its recent performances against Afghanistan and India in the subcontinent and against Ireland at home - playing consistent, smart cricket to win the series. Sri Lanka are a very good team in their own conditions. I do not expect it to be easy but our team has shown that it is very capable."
West Indies squad to play Sri Lanka: Fabian Allen, Sunil Ambris, Darren Bravo, Roston Chase, Sheldon Cottrell, Jason Holder, Shai Hope (wk), Alzarri Joseph, Brandon King, Keemo Paul, Kieron Pollard (captain), Nicolas Pooran, Rovman Powell, Romario Shepherd, Hayden Walsh Jr.