Wayne Bennett says the lack of preparation and no mid-season international were partly to blame for the disappointing results on the Great Britain tour.
The Lions went down to their fourth successive defeat on the tour, losing 28-10 to Papua New Guinea.
Bennett said: “We missed the mid-year Test this year. We need to play in that, that’s where these guys are getting all their improvements from.
“Australia come in with an Origin Series behind them, New Zealand come in to play us after they’ve played Australia at the end of the season. Everyone is doing more preparation than we are.
“If we want to compete with them, we’ve got to get ourselves back to that level.”
It was the first time since 2016 that England hadn’t played in a mid-season test, having taken on Samoa and New Zealand in the past two years.
Bennett also said that GB’s defeats on tour show how the standard of international rugby league is improving.
There are increased calls for investment in other European countries, such as France and Wales, to provide capable competition for England, with the ongoing growth of the Pacific Islands, notably Tonga, giving the international game a shot in the arm.
He added: “You all need to have a good look at what’s happened with the international game over the last five or six years.
“They’re playing more Test matches and more competitions and you’re starting to see the benefits of it.
“You’ve seen the emergence of Tonga, you’ve seen how much your team (PNG) has improved and Samoa are much improved. The boys all want to play for their country now.
“So, while we’ve got to swallow a bitter pill as Great Britain, on the other side of the ledger I see the game getting stronger.”