The former Everton defender believes that Vicent del Bosque's persistent decision to not use a target man in his side's formation has made it easier for their opponents
In a formation reminiscent of Barcelona under Pep Guardiola, Del Bosque has decided to play with a false nine, predominately either Cesc Fabregas or Andres Iniesta, to little avail.
Spain managed to win Group C regardless, although lacking conviction of previous tournaments, as they struggled against both Italy and Croatia.
"People are going crazy about Spain, but the world moves so fast that we forget the detail," said Bilic.
"They have looked vulnerable, not only against us but also against Italy.
"For me, it is easier to defend against Spain when they are playing without [Fernando] Torres, [Alvaro] Negredo or [Fernando] Llorente.
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"To me, they are more dangerous when they play with a striker. Why? Because when they don't have someone in the box, they lack depth.
"When you play against Torres, you can't push up. They can't stretch you without a striker.
"I am not sure if they are missing David Villa, they have enough quality strikers on the bench. I would have preferred them to play without a striker against us because it makes it easier - you just have to protect the wings and it is much easier."
After winning the group ahead of Italy, Spain now play France on Saturday, June 23 for a place in the semi-final as Del Bosque's side look to retain the trophy.
Bilic will now coach Lokomotiv Moscow and will be succeeded by Igor Stimac in the Croatian national team.