Arsène Wenger is surprised at the proposed sale of Juan Mata from Chelsea to Manchester United - because it would strengthen one of the Blues' biggest rivals.

According to reports, the Spanish playmaker will complete his move to Old Trafford this week after Chelsea accepted a bid from United. Wenger admires Mata and says the imminent deal poses questions about the January transfer window.

"I am surprised because Juan Mata is a great player and Chelsea sell a great player to a direct opponent," said the Arsenal manager.

Some teams have already played twice against one opponent and some others not. I think if you want to respect the fairness for everybody exactly the same, that should not happen

"It opens again a little bit the opportunity of this transfer market because Chelsea had already played twice against Man United so they don't play again.

"They could have sold him last week but it opens at least the opportunity [to question] the dates of this transfer window. Some teams have already played twice against one opponent and some others not. I think if you want to respect the fairness for everybody exactly the same, that should not happen.

"I just think you would want that everybody is on the same level. Chelsea do not make the rules but maybe the rules should be a bit more adapted to more fairness."

Mata will arrive at United in the wake of their shock Capital One Cup defeat to Sunderland. Former Gunner Vito Mannone was the Black Cats' hero after saving twice in a penalty shoot-out that saw just three of the 10 spot-kicks converted.

"Look, we have all played football and we have all missed penalties," said Wenger. "It's more down to the pressure than the ability of scoring penalties.

"You have seen players missing penalties last night, but if you get them today in front of the goalkeeper in training they will score. At the moment the players feel under pressure at Man United and then it's more difficult to score the penalties."

Wenger also feels that United manager David Moyes has the strength to deal with his new club's difficult spell.

"You have to survive massive disappointments and keep your belief high in what you do because to win is easy for everybody," said the Frenchman.

"How to live with defeat, that is what managers are about, and David Moyes is strong enough. For me he deals with the situation very well.

"They go through a difficult moment in their season but they can still come back. They are in a strong position in the Champions League and they just bought a great player, so Man United has still to be considered a threat."

Copyright 2014 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source Chris Harris 23 Jan 2014