The tourists romped home by eight wickets with nearly eight overs left here at the Riverside, captain Mahela Jayawardene scoring an unbeaten 126, to secure the NatWest Series trophy by going 3-0 up in the five-match encounter.
Admittedly England were without six senior players, including key all-rounder Andrew Flintoff and regular captain Michael Vaughan, but that was no excuse for the succession of short and wide deliveries they served up to the Sri Lanka batsmen.
England have yet to win the World Cup and their chances of triumphing in the West Indies looked a long way off after an eighth defeat in their last 10 One-day internationals.
"Every game between now and the World Cup is vitally important," Strauss told reporters. "Sure we have injuries and there are players to come back but you don't know who's going to be injured come the time of the World Cup.
"The guys that are not necessarily in our first-choice XI at the moment, they've got to show they are up to world-class standard and so far we haven't done that.
"There's no place to hide in international cricket and this is where you see guys' real characters.
"We'll learn a few things about some players in the next few days," said Strauss, one of four England batsmen who fell in the 30s in an innings where Ian Bell's sluggish 77, a testament to the Sri Lanka attack's greater discipline, was the top score.
"We've still got 20 or so games to go the World Cup, plenty of time to build-up some winning momentum but you've got to start sooner rather than later."
Although England only sent down five wides at the Riverside, compared to the 43 in total conceded during the 20-run and 46-run defeats at Lord's and The Oval, they yet again allowed Sri Lanka to get off to a flyer with wayward pace bowling from Stephen Harmison, Liam Plunkett and Sajid Mahmood.
Prior to that, Strauss and Marcus Trescothick put on 62 - their best opening stand of the series.
But England then lost three quick wickets to hand Sri Lanka an initiative they never looked like losing.
"The main problems have been the same problems all the way through the series - no batsman getting a hundred and from the bowling point of view we didn't hit our straps early," Strauss explained.
"That enabled Sri Lanka to get off to a flyer and once they got into that position they didn't have a lot of pressure on them."
Strauss insisted his pace attack had it within them to bowl a tight line. "I've seen the guys in practice and they bowl straight and I've seen them for their counties and they bowl straight.
"To me, it's a mental thing. You've got to give the Sri Lankans some credit for applying the pressure on our bowlers but we've got to find ways of counteracting that."
Jayawardene, who faced just 127 balls with 18 fours, was understandably elated as his young team sealed a rare triumph away from the sub-continent.
It was their fourth straight victory over England, including a Twenty20 success, after they'd battled back to square the Test series at 1-1 earlier this month.
All tour they'd been without regular captain and senior batsman Marvan Atapattu because of a back injury and for this match they were minus Muttiah Muralitharan, who'd left the squad to be with his ill son.
But Sri Lanka looked anything but an under-strength side.
"That was probably the best game we've played out of the three," said Jayawardene. "I think our bowlers bowled really well and when chasing a score it's important one of the guys makes a big hundred and it was my day here.
"This is very big. The whole tour has been hard for us. When we came here a lot of things were said about us being a young side but our guys worked really hard and it's paying off now."
The series continues at Old Trafford on Wednesday and concludes at Headingley this Saturday.