England delivered a statement performance to subdue the Pumas 27-10 in Marseille on Saturday, washing away their miserable three defeats in four matches from August.
Defence coach Sinfield hailed the team for restoring two of Steve Borthwick’s manifesto pledges: fight and spirit seen as a minimum.
Head coach Borthwick had stopped citing those qualities across a dispiriting warm-up programme, comprising damaging defeats by Wales, Ireland and Fiji.
But now Sinfield believes those attributes are back, after George Ford’s three drop goals and six penalties handed England the ideal start at the Stade Velodrome.
England’s only copybook blot was the high-tackle red card for Tom Curry, who now faces a disciplinary hearing in Paris on Tuesday night, where his sending off could be contested.
“I think we showed a bit of what we’re about, but I still feel there’s loads of improvement in us,” said Sinfield. “I still think there is more in us.
“It was pleasing to see some aspects really improve, but I think probably more so for the players. We need to ensure we get more of that, and keep moving forward in each of our training sessions to put our best foot forward.
“We knew that performance was coming, and to have to do the vast majority of it with 14 men shows the fight and the spirit that we have spoken about for some time now. That hadn’t transferred onto the field for us before.
“So, we knew it was going to be a challenging game for us, we also knew that Argentina are a fantastic team, and are going to cause some teams some real issues at this World Cup. But we got through it, and we took a step forward.
“I was disappointed for us to concede the try at the end, but I thought overall we put in a really professional job.”
England will now face Japan in Nice on Sunday, with designs on topping a pool also containing Samoa and World Cup debutants Chile.
Ford dominated against Argentina, with his three drop goals in seven first-half minutes turning the game in favour of 14-man England.
Sinfield was running the tee onto the field for Ford, so had a close-up view of the Sale star’s superlative performance.
“I thought it was a masterclass, he really grabbed hold of the game for us,” said Sinfield. “I thought he was faultless with the boot. I think he brings a calmness to us.
“You want your leaders to step up in a variety of different ways in big games, and George certainly did that.
“I think his relationship and partnership with Freddie Steward at the back was special as well, in a different way.
“I know we’ve discussed Marcus Smith and his role as a 15 for us, but I thought the pair of them were outstanding.”