The 30-year-old striker has been disillusioned with life under David Moyes at Old Trafford and spoke of his frustration with his team-mates after Tuesday's 2-0 defeat to Olympiakos
Goal reported on December 14 that Van Persie was considering his future with the Premier League champions as he continued to be disillusioned with life under manager David Moyes.
The Dutch striker has been criticized for his anonymous performance in the Champions League defeat to Olympiakos on Tuesday, where the 30-year-old missed a crucial chance to halve the deficit late on in Greece.
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Krol, who appeared 83 times for his country, believes that the former Arsenal star is showing signs that he is not comfortable at Old Trafford.
"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," he told Goal. "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."
Van Persie is contracted to United until 2016, having joined from Arsenal for 24 million pounds back in 2012, and has scored 14 goals in all competitions for the Red Devils this season.