England blew a 2-0 lead to draw 2-2 with Iraq in their opening group game.
Former Everton defender Dier said: "I think we're prepared, we just need to play our own game and let them worry about us.
"Everyone was disappointed on Sunday. To be 2-0 up and then to concede two goals, sloppy goals, was disappointing.
"In the first half we were really good, we passed and kept the ball really well.
"We created loads of chances and could've killed the game off.
"So we need to look at that first half and take the positives from that."