Sven-Goran Eriksson blamed the heat for England's lacklustre second-half display during the 1-0 victory against Paraguay on Saturday.
An early Carlos Gamarra own goal ensured England got their World Cup campaign off to a successful start in Frankfurt, but they looked anything but contenders to win the tournament in the second 45 minutes.
Asked if he expects his side's performances to improve, Eriksson told BBC1: "I hope so. You always try to do better but I'm happy today. We can play much better but I'm happy.
"We started the tournament with three points and it's very important.
"Maybe we suffered a lot. I think this was a real hot, hot day and I think that knocked us in the second half."
Captain David Beckham, whose free-kick led to the goal, backed Eriksson's view.
"You don't realise how hot it was out there," said the Real Madrid midfielder.
"We were good at times but we gave away the ball too much. But it was hot and we got what we wanted. That was a good start and we have the three points.
"The performance could be better but we will work on that. We set out to get the three points and we did that.
"We let them (Paraguay) pass the ball around too much.
"But we were exhausted. We can put it down to that because we are usually strong and our fitness levels are high."