The tourists took an unbeatable 3-0 lead in the five-match encounter with a crushing eight-wicket win here at the Riverside, arguably the most emphatic of all their conclusive victories in the series thus far.
But well though Sri Lanka played, they were assisted mightily by yet more wayward England bowling at the start of the innings and the renewed failure of one of the home batsmen to make a match-winning score.
Defeat was England's eighth in 10 One-day internationals and, with time running out until next year's World Cup in the Caribbean, Fletcher said it was down to the experienced players to end his injury-hit team's dismal run, starting with this week's fixtures at Old Trafford and Headingley.
"They did it against Australia, the best side in the world, but it's up to the top four to go out there and get a hundred," Fletcher said.
Excluding the recent match against minnows Ireland in Belfast, England have not scored a One-day international century since opener Marcus Trescothick hit an unbeaten 104 in a nine-wicket win against Australia at Headingley last year.
"It's a mental thing. If you get to 30, technically you can't be lacking something and mentally you have to get through the session and get those scores of 100," Fletcher added.
Just as concerning for the former Zimbabwe captain was the Riverside form of pace trio Stephen Harmison, Liam Plunkett and Sajid Mahmood, all of whom gifted Sri Lanka boundaries with a succession of short and wide deliveries.
"At practice they bowl straight," said Fletcher. "We put markers down and they hit those markers on a regular basis but you have to give some credit to some experienced Sri Lankan players who can put our youngsters under pressure."
England have a lengthy injury list with captain Michael Vaughan (knee), key all-rounder Andrew Flintoff (ankle), Ashley Giles (hip), Simon Jones (knee) and James Anderson (back) all out of the Sri Lanka series.
The question of whether Vaughan, who led the team to Ashes glory last year, will need fresh surgery on his right knee will be discussed at a meeting of Yorkshire and England medical staff later Monday.
"All of our injured players are world class - Vaughan, Flintoff, Giles and Jones - and it's going to be a big concern if they don't come back and play this season," said Fletcher, whose team face a potentially gruelling four-Test series against Pakistan starting at Lord's next month.
Vaughan, worryingly for England, was unable to field for two days of Yorkshire's County Championship match against Sussex at Arundel last week because of renewed stiffness in his knee.
"It's a concern. We thought he was coming back and there was a period when we thought it wouldn't be long before he was back. But after this injury they are going to have to re-assess it," Fletcher admitted.
Neither Vaughan nor Giles, ever-present in the 2005 Ashes, have played for England since the series defeat in Pakistan late last year.
One consolation for Fletcher is that Flintoff appears to be on course to play at Lord's after being told to rest his ankle.
"He's lost quite a lot of weight and he's working hard in the gym so there's been some good reports of him," explained the coach.
And there was also encouraging news cocerning Kevin Pietersen, hit on the left knee while batting for the second time in as many matches at the Riverside.
However, a scan Sunday revealed no fracture only heavy bruising and England have retained the South Africa-born batsmen in their squad for the concluding One-dayers against Sri Lanka.