Iestyn Harris wants his Wales rookies to cut out their naivety ahead of their Wembley showdown with defending Four Nations champions New Zealand.
And Harris knows Wales, mainly made up of youngsters and part-time players, must improve in Saturday's clash.
"We're learning how to compete at this level and intensity," said Harris.
Wales qualified for their first Four Nations tournament, international rugby league's annual showpiece, by winning their second successive European Cup.
But England quickly highlighted the step up in class as the Welsh went behind inside the first minute as Sam Tomkins scored the first of his four tries at Leigh.
"I am proud of my players that they stood up to mental and physical challenge," said coach Harris.
"But we have some improvement to make on game management and some individuals errors to cut out.
"We lacked that ability to build pressure on England and make them feel a little bit edgy. When we did, we looked dangerous but we didn't have enough.
"We just had a couple of key individuals that went away from it slightly, which we'll work on.
"There's a little bit of naivety in our group because they are not hardened at this level and that comes with playing at this intensity.
"Sometimes you have to have trust in yourself and what we're telling them.
"We stood up well and put a lot of effort into our structure. They had trust in our structure and it got us out of trouble at some stages.
"My players are prepared to put their bodies on the line but we must be a bit smarter with it.
"If we can add a bit of smartness with our heart, passion and direction, then we'll improve."
Wales will play their first rugby league test at Wembley on Saturday for 78 years and Harris, the former Crusaders coach and newly-appointed Wigan assistant, has a promise for those Welshman that train well in the build-up.
"The players are excited at the prospect at playing at Wembley," said the former Great Britain stand-off.
"However we'll have a look to see who trains at the level we need them to train at.
"We won't pick the team just on performances on the field but performances in training."