Everton were beaten 2-0 by Manchester United in their most recent match.
Sigurdsson was left on the substitutes’ bench, as Everton fell to a 2-0 defeat against United.
The Toffees carried little attacking threat throughout the contest, and Souness admits he doesn’t understand why Allardyce didn’t use the Icelandic international.
“It’s a bit of a mystery why Sigurdsson didn’t come on at any point during the game, because he’s a bit of a threat,” Souness said.
“He’s the biggest threat they’ve got. I understand Sam’s thinking, when you don’t have the ball he can be a passenger, but when you’re a goal down you might as well lose by two or three. That was a mystery for me.”
Everton simply could not break down a resilient United back line, as their attacking struggles continued.
The Toffees have now failed to win any of their last four matches, with their form slipping again.
Sigurdsson has actually been in impressive form of late, but Allardyce clearly did not feel that the former Swansea City man was suited to yesterday’s contest.
Everton are next in action on Friday evening, when they take on Liverpool in the FA Cup.
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