This is actually his problem - he aimlessly runs into areas he often can't receive the ball in. He closes down space fairly well when the opposition have the ball, but that's it.
Nobody is doubting that he doesn't jog around a lot. Indeed, that's the problem - he jogs around a lot. If he's a second striker, fine - concentrate on getting into areas to support the striker and get into the area. If he's a playmaker, fine - move into space, demand the ball, dictate play. The problem is he does neither of these things.
Here's my question for everyone - what does Sigurdsson do during a game? What does he do to consistently influence the play?
The answer for me to that question is "next to nothing". He's a talented footballer who seems constantly caught in between two minds as to what he's supposed to be doing, and therefore does nothing. Occasionally, his raw talent will pick out a blinder of a pass or an astonishing goal - but for the vast majority of the game he's a passenger as he seems tactically lost.