Gareth Southgate said England “put down a great marker” by thrashing Iran 6-2 in their World Cup opener but “must be better” in their final two group games.
Bukayo Saka scored twice as the Three Lions impressed in attack, but Mehdi Taremi’s two goals for Iran highlighted a need for defensive improvement.
England face USA in their second Group B game on Friday.
“It’s a great start but we’re going to have to be better,” England manager Southgate told BBC Sport.
“To win by that margin – to play as we did for the majority of the game – we’ve got to be really happy. That’s a great marker to put down.
“But we shouldn’t be conceding two goals at that stage of the match and we’ll have to be right on our game against the States.
“We had, I think, 24 minutes of added time across the game, so it’s a long time to focus – but we just lost concentration and when we play at a slower tempo, we’re nowhere near as effective.
“I’m a miserable so-and-so and I should be more excited, but I’ve got to keep the team on the right track and we will have to be better.”