Maguire is really a horrible player. His defending has been shit for Manchester United and I really ask myself to date why they spent so much in that player. He even isn't really worth above 20m Euro. I hardly think his absence would be felt in this upcoming match against Iceland but Iceland are also capable of upsetting big teams.
As a Liverpool supporter, you have no idea how much I loved every moment seeing Maguire's confused face look when he does a blunder. But as a neutral supporter watching him play for England at the last World Cup was actually very nice!
I actually don't mind Maguire for United or England. Granted he is not the best technician or speediest defender out there but he reads the game quite well and he will get all the air duels in the match. It is up to the manager/coach to organize the defense to suit him. You have to push opposition players to the flanks and then he will make good work of all those crosses. If you leave him one on one with fast players he is doomed