The Chile international has joined the Rs on-loan from Juventus.
Redknapp said: "It's been a problem for Isla in so much as he's a wing back when he was at Juventus.
"I brought him into play in the same position as well. We had a couple of results that didn't go our way early on, so we went to a four and he's not been the same player as we were in a five."
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