Chasing a challenging total of 312, the Proteas were bundled out on 207 inside 40 overs.
"This is where it's important for us to make sure that you look at the World Cup for what it is. You know, you`re going to play games. You're going to come up against quality opposition. England was better than us in all three facets of the game. They show why they are such a good cricket team, and now we have to look at the league. It's a league competition," ESPNcricinfo quoted du Plessis as saying.
"As disappointed as we are with our performances today, you have to make sure you learn from it quickly and put every single dart to bed because there is the next game and you have to make sure you play good cricket again," he added.
Having known that England have a long batting line-up, South Africa restricted the host for 311/8.
The Proteas conceded just 76 runs in the last 10 overs, which made it seem the du Plessis-led side had won half battle.
One of their go-to bowlers, Lungi Ngidi picked up three wickets, including ending the innings of Ben Stokes for 89.
Veteran leg-spinner Imran Tahir and Kagiso Rabada returned with two wickets each.
"Yeah, I thought after the first half, that we did a good job with the ball. You know, as I said before this game, England more so than any other team in this World Cup, bat longer than anyone else. So if you can stop them with the long batting line-up you have, then you`re doing a great job with the ball," the South Africa captain said.
"For them to score as little runs as they did the last 15 overs and us picking up as many wickets as we did, was a great achievement by our bowling units. But yeah, as I said, 310 is still a big score, so what you need to do is you need to start well, and we didn`t have that luxury," he noted.
The chase looked achievable until Hashim Amla retired hurt after getting hit by a Jofra Archer bouncer.
The setback disrupted the South Africa batting and a wobble took place too before Quinton de Kock held his nerves to rebuild the side.
In the 23rd over, Liam Plunkett dismissed de Kock which broke the backbone of South Africa.
Amla returned to bat but it was too late as his side was at 180/7 and eventually South Africa folded.
"When you are chasing a big score like 310 what you need is a good start. You need to make sure that you settle the bats in the dressing room with a good partnership, whether it's zero down or one down, but a good start is really important and obviously with him (Amla) coming off, and then a few overs later, Aiden (Markram) got out and then one or two overs later, I got out. When you go three-down on a big score like that, you're making it really tough for our batting unit to perform. And then the score just looks big. It looks too far," du Plessis said.
Here's the complete fixture of Cricket World Cup 2019:
South Africa will take on Bangladesh in their second match on June 2 at the Oval.
South Africa were handed a 104-run defeat at the hands of Eoin Morgan-led England in the curtain raiser of the 50-over tournament at the Oval