Arsene Wenger last night taunted Manchester City by claiming Robin van Persie made the right choice to go to Old Trafford.
Wenger admits it is "painful" to see van Persie in a Manchester United shirt but claims the in-form Dutchman is the reason why City are seven points behind and could miss out on the title.
The Arsenal boss believes van Persie would have got lost among City's wealth of strikers but the 16-goal striker is thriving at United because he is their big star.
Wenger said: "You know at City they have plenty of world class strikers. Tevez, Dzeko - hopefully they won't have Aguero this weekend. They have the quality. If he (van Persie) had gone to City he would have been just another striker in a bunch of world class players.
"He is at the peak of his game, 29 going on 30. He has gone through difficult moments in his career, he knows his game. He uses all his qualities in a calm and intelligent way.
"What is painful is to see Manchester United so far in front of us. You know when you sell Robin van Persie to Man United that will be the case.
"He is one of the best strikers in the world and you know he will score goals for them. He got 30 for us in the league last year."
There has been no love lost between Arsenal and Manchester City down the years as a steady stream of players - Kolo Toure, Gael Clichy, Emmanuel Adebayor and Samir Nasri - have all headed north to the Etihad.
But van Persie always made it clear that he did not want to go there and preferred United or Juventus which may have been as a result of previous bad blood as Adebayor even famously stamped on his head in a particular feisty clash.
Wenger also admitted that, as Mirror Sport revealed at the time, he finally agreed to sell to Sir Alex Ferguson after an emotional phone-call in August while he had previously promised that he would not sell to City.
"It was a very professional phone-call, and there was more than one," said Wenger. "I will tell you the rest another day.
"But the title race is still very open. When you watch the games, no team today goes into a Premier League game and is sure to win it. Therefore I believe every team can drop points.
"It is much more open than people think it is. You see Man United as well, they win their games because they score many goals, but it is difficult for everybody."
Wenger's hope had been selling van Persie would allow him to make his squad stronger all round but even the additions of internationals Santi Cazorla, Lukas Podolski and Olivier Giroud have not helped solve Arsenal's lack of consistency.
The Frenchman believes beating City - who have not won in the league at Arsenal since 1975 and have not managed a goal since DeMarcus Beasley scored in a 3-1 defeat in 2006/07 - would lift their confidence and belief for the run-in.
Wenger added: "It would help to improve the confidence of the team. That is very important. You can see that for every team it is important. For Chelsea the other night when they started to miss chances, it has an effect on the confidence of the team.
"That's why at home are in a similar situation to Chelsea, overall positive results away from home and at home we are getting back now to positive results. We beat West Brom and Newcastle in a convincing way, so it is important to continue that.
"We know it's a vital period for us. We are on a positive run. We still lack a bit of confidence in some situations.
"I feel our team will be very dangerous if we have full confidence. We are not far from it but we need another positive result in a big game to be completely confident and believe in our ability to come back."