Sigurdsson is not a bad player; he's simply a bad player in that type of team.
It is no coincidence that he has looked decent this season when Richarlison and Walcott have played either side of him, as they push teams back with a fear of their trickery and pace respectively.
But as soon as you take Richarlison away and have a blunt instrument like Calvert-Lewin instead, teams can push up five yards and come more central and Sigurdsson is totally ineffective.
He's not a playmaker; he's an exploiter of play when good, nimble footballers are doing the work around him.
Today, we should have dropped him deeper alongside Schneiderlin as more of a central midfielder who can step forward, and played a front two instead. Silva is 100% responsible for tactically throwing that game away today. How many times Everton have to make the same mistake of having two crabs in midfield and expect miracles is beyond me.
As for Gylfi's long term future at Everton, I actually think he has one - but it'd mean building a side with depth so that we don't have DCL on the left wing ever again. We were simply unfortunate that Bernard and Richarlison were both out.