The former Arsenal star has struggled to find consistent form with the Red Devils this season, mainly due to injury, but Krol suggests he may be having a difficult time at Old Trafford.
In comparison to last season where he netted 30 goals in all competitions, van Persie has scored 12 this campaign and although that has been blighted by injury niggles, Krol feels he is not at the top of his game.
"I watched him play against Olympiakos the other night in the Champions League and he failed to put away a chance that he would normally score blindfolded," the former Oranje and Ajax defender told Goal.com.
"Then his reaction later spoke volumes. He is not settled - actually the whole team is lacking self-belief.
"Nobody doubts the player's qualities or what he can do with the ball. Such crisis moments occur once in a while but van Persie will definitely come good. Holland needs him for the World Cup."
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