The Netherlands international endured an injury-hit campaign with United, making just 18 Premier League appearances due to injury.
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The striker is in Brazil with Louis van Gaal's squad preparing for the World Cup, but has admitted he became frustrated by the amount of time he was forced to spend on the sidelines last year.
"Over the last few years I put down the best series of my career. I played 93 matches in the Premier League in a row. Most of them as a starter, some as sub. I never thought about injuries," he told Metro Holland.
"But this season I had three, where I was out for at least a month every time. Sometimes I got despondent of it. It started with something small.
"I had a broken toe and because of that I started to walking different, which had consequences for the rest of my body."
Netherlands begin their World Cup campaign on Friday with a clash against holders Spain and the Manchester United man says he needs to improve on his goalscoring record in the tournament which he feels is disappointing.
"I've scored two goals during a World Cup and those numbers aren't pretty. I want more, more more goals," he added. "The World Cup can finally start."