MATCH REPORT: Moeen century can't save England against Sri Lanka
In the opening clash of a seven-match series in Colombo on Wednesday, Sri Lanka's opening pair of Tillakaratne Dilshan and Kusal Perera got off to a strong start, putting on 120 for the first wicket inside 23 overs.
The hosts eventually totalled 317-6 and, with the highest successful ODI chase in Sri Lanka standing at 289, England had their work cut out.
A brilliant 119 from Moeen Ali kept up with the rate, but the tourists lost wickets at regular intervals and even Ravi Bopara's 65 could not prevent England falling to 292 all out with 17 balls remaining.
"The first 10 overs we didn't bowl as well as we should have done," said Cook in the post-match presentation.
"We dragged it back nicely but in the last few we always tend to leak a few more than we'd like. Moeen played a blinder but we lost too many wickets which you can put down to lack of match practice.
"We know if you get behind the rate it's difficult. Mo showed us the way.
"It's frustrating because we're not putting a whole game together. We thought 300 was a par score on that wicket and our [17] wides have cost us but there's a lot of positive things from the way we attacked Sri Lanka."
Victorious skipper Angelo Mathews, meanwhile, was understandably delighted with the efforts of his team.
"It was a good game of cricket," he said. "We could tick all the boxes.
"The fielding was outstanding - we highlighted that after the India tour. We still can improve but I though our batting was brilliant.
"We got Kusal Perera back in form and Jeevan Mendis played really well too."