The first eight teams in OID rankings by September 30 will qualify automatically, and a qualifying round will be held to select two other teams.
With the 3-2 defeat to Zimbabwe, Sri Lanka lost five points and is ranked eighth with 88 points, just 10 points above West Indies.
The points difference between the two teams before the defeat was 16 points.
Earning a point after a win against India, West Indies will have six ODIs against England and Ireland and they will have Chris Gayle and other senior players rejoining the team.
Sri Lanka will face a five-match ODI series against India starting on August 20.
If the West Indies are able to overcome England and Ireland, their points will go up by nearly 10 points.
On the other hand, a defeat at the hand of India will mean Sri Lanka will lose up to five points, relegating it to the ninth place in the points table and denying automatic selection for the World Cup.