Arsene Wenger still feels the pain of losing star striker Robin van Persie to Manchester United Any mention of Van Persie's loss, with United already 18 points clear of Arsenal in the Premier League, seemed almost to trigger an involuntary shudder in Wenger. Was it painful, he was asked, to watch the Dutch striker continue his wondrous form from last season for the club's arch rivals? "What is painful is to see Manchester United so far in front of us," he replied. We knew when we sold him to United that that would be the case. He is one of the best strikers in the world, so you know he will score goals for them - he scored 30 for us last year. Recalling the fateful sequence of telephone calls with Ferguson that sealed Van Persie's departure to Old Trafford last summer, Wenger said: "It was a very professional phone call, and there was more than one. I will tell you the rest another day. Less mysteriously, the Frenchman explained how Van Persie had learnt to be a far more lethal target man, scoring 20 goals in his 26 appearances so far for United. At almost 30 he is at the peak of his game. Now he uses all his qualities in a calm and intelligent way. I know him very well, and he has a high level of confidence in his qualities. He is more patient than he was five or six years ago. He stays up front, and you won't see him for maybe 20 minutes. Before he used to come back into midfield, but he uses his killer instinct much better now. Van Persie's prolific form at United has ratcheted up the pressure on Wenger to keep Theo Walcott at the Emirates, and the manager confirmed that he was "more optimistic" that the striker's contract talks would reach a successful resolution. While the terms are understood to fall below the £90,000-a-week deal that had been speculated, Wenger believes Walcott will sign by the end of this month.