Lancaster's side were edged out 22-17 by the Springboks in the opening game of their three-match series at Durban's Kings Park.
The challenge now gets tougher with the second Test taking place at altitude in Johannesburg and the Springboks' limited preparation time no longer a factor, but Lancaster insists players are far from disheartened.
He said: "The boys are disappointed because they lost. They want to win, that is the primary emotion at the moment.
"But when we look back on it, at half-time we were definitely in a positive mindset and we felt we had the better of the first half, even though it was only 6-6.
"The boys are certainly very positive and the good thing is we have already seen one or two areas we can fix up.
"The players are talking about it already and that is a good sign. They are not downcast and they are feeling there is no chance of turning up next week and not winning.
"If anything, this will be their motivation to right the wrongs we did in this game."
England fought hard in the first half and twice led with Owen Farrell penalties but both were cancelled out by Morne Steyn.
The Springboks upped the tempo after the break and relentless pressure eventually told as Steyn and new captain Jean de Villiers broke through for tries.
England responded with two more Farrell penalties but another two from Steyn put the game beyond them despite an impressive late consolation from Ben Foden in the corner.
Lancaster said: "I don't recollect too many clear-cut chances that we created but we definitely created some and put pressure on the Springboks.
"But they defend with pretty much everyone in the line, it is hard to break them down.
"Composure was probably the key word that we need to work on - make sure we play in the right place at the right time but, equally, when we have got opportunities to get the ball off the field we get it off the field.
"Once they were across the gainline, irrespective of how we slowed the contact down or got around the corner, eventually the hard-running South African forwards made a dent into us.
"We need to work defensively and make sure we win that gainline battle. That is crucial for all sides."
The spirits of the England side revived under Lancaster remain intact.
Lancaster has transformed the national side after last year's dismal World Cup and feels the new faces he has introduced will be better for the experience.
Forwards Joe Marler and Tom Johnson both worked hard on debut while full-back Mike Brown shone on his return to the starting line-up, although he has a thumb injury which will be assessed.
Lancaster said: "It is about building experience.
"We had four or five under-21s in the side.
"For them to come here and play at this level and this intensity - I thought they stepped up remarkably well.
"Mike played as he did for Harlequins throughout the season. He didn't just look safe under the high ball, he was good. "He has been a long time waiting for his chance and I am really pleased with how he played."
South Africa-born centre Brad Barritt is England's other injury concern after leaving the field in the second half with a facial injury.
The players who did not feature will get their chance to stake a claim for a place in the second Test in a tour match against the Southern Barbarians in Kimberley on Wednesday.
Lancaster said: "We need the midweek boys to pitch up and play well, put pressure on us and make sure we have got some good selection decisions to make.""
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