Afghanistan spinner Mohammad Nabi took three wickets in a single over to leave the Lankans struggling at 149-5 and Jayawardene said the string of soft dismissals was cause for concern.
"The way Sri Lanka lost wickets... in the middle order is a real concern and there were too many soft dismissals," he wrote in a column for the International Cricket Council.
"Looking at the wicket... this was never going to be a high-scoring game. However, there's no question that Sri Lanka could have batted better. They should have been looking at 250 plus."
Lankan coach Chandika Hathurusingha said his batsmen were suffering from a crisis of confidence that had been exacerbated by the bowler-friendly conditions.
"When a batsman is in it looked an easy wicket to bat on but we lost one wicket and then we lost three wickets in one over," Hathurusingha said.
"There were a lot of tentative shots, that was the cause of some of the dismissals. We're lacking in confidence... we allowed them to come into the game."
Sri Lanka arrived at the World Cup in patchy form, losing two warm-up games against South Africa and Australia before being beaten by New Zealand by 10 wickets in their first match of the tournament.
Against Afghanistan, they needed pace bowlers Nuwan Pradeep (4-31) and Lasith Malinga (3-39) to help guide them over the line and Hathurusingha said getting their first win would help build confidence for the rest of the tournament.
"This gives us a lot of confidence because we really needed a win," he said. "We haven't had much success lately. I'm sure you will see a better team, a better approach from our team after this win. We needed this win badly."
Sri Lanka next play Pakistan on Friday.
(Reporting by Rohith Nair in Bengaluru; Editing by Simon Jennings)
