The Belgium defender acknowledged that the Premier League top scorer was an integral part of the Arsenal team last season but insisted that the club must now switch its focus to the start of the Premier League campaign.
Van Persie is set to undergo a medical and finalize personal terms at Old Trafford on Thursday before the move is completed ahead of the division's opening weekend.
"I am disappointed because I wanted him to stay at the club," the Vermaelen told Sky Sports News.
"He was fantastic for us in the last year, scored lots of goals and I am disappointed he is going, but we have to focus on the next season with Arsenal. We have to fight and I am confident we have a squad to win something."
The 26-year old was relieved that the issue had been dealt with before the Gunners' season had kicked off against Sunderland on Aug. 18."We start Saturday and this is all clear now, Robin is leaving and we have to move on," he concluded.
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