Hislop believes Redknapp's decision to ignore Cole's off-the-ball development came to everyone's disadvantage.
The former Hammers goalkeeper told the Daily Mail: "Harry kind of liked Joe just to express himself, and we'd play around him, or around Joe's unpredictability.
"Encouraging him to be himself didn't help him. When you sign him you identify that, talented though he is, when we don't have the ball he's got to give us something else.
"Everything about his game on that side, when your team don't have possession, he simply wasn't good enough. Harry did not help him in that respect."