The Spurs midfielder struggled for England last in their 0-0 draw with Slovenia.
Dier gave the ball away several times and was also booked in a torrid first half performance.
England player ratings against Slovenia
Tue, October 11, 2016Express Sport brings you England's player ratings from their World Cup qualifier against Slovenia
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Express Sport brings you England's player ratings from their World Cup qualifier against Slovenia
At club level Wanyama appears to be ahead of the 22-year-old in Mauricio Pochettino’s starting XI at present.
And Redknapp feels Dier is struggling to deal with the competition.
"Eric Dier is not the player he was last season," Redknapp told the Daily Mail.
"There was almost a strut to him last year with Tottenham and England, but I feel Victor Wanyama has had an effect on him.
"There's that little competition for places and he is slightly lacking in confidence."