He played centrally in the role behind the striker - but Argentina play a 4-4-1-1 not a 4-2-3-1, so he was often pushing up alongside Finnbogason as he did in the game vs Argentina.
I think that's his best role in that particular system.
He pressed when Iceland didn't have the ball, as the front men in Iceland's system are tasked with pressing and narrowing the passing angles.
But whenever Croatia got through that initial press, he'd drop back into the midfield and be on hand to pick up the ball and play the pass on the counter.
If Iceland had the ball from the back, he'd often also drift out wide and get into a crossing position - which is where a fair few of the chances he created came from.
Problem is and always will be until we change the system is that, we don't play like that with a support striker/number 10 off a striker. It's been 4.3.3 for 2 seasons (and why I still can't get my head around why we bought him).
At Watford Silva played with wing backs in a 3.4.3, with one sitting DM next to a CM and 3 attackers.
Gylfi isn't a CM as he doesn't have the vision or able to dictate play. He can't, apparently, play wide of a forward 3 either
So unless Silva figures something out to get the best out of him, he could struggle.
For me, if we're playing a front 3 it's Lookman, Cenk, and Walcott starting. So Gylfi is looking to be change his game up.