The Arsenal manager, Arsène Wenger, has voiced surprise that Chelsea could sell their playmaker Juan Mata to Manchester United and hinted that the transfer is unfair for teams yet to play United twice in the league.

The Spaniard is expected to undergo a medical at United ahead of a potential club record move to Old Trafford and possible debut against Cardiff next Tuesday. Mata, 25, has fallen out of favour at Chelsea this season but Wenger is still amazed he could be allowed to join a rival. Asked if he was surprised, Wenger said: "Yes I am because Juan Mata is a great player and they sell a great player to a direct opponent."

Wenger does note, however, the timing of the move with Mata now unable to affect Chelsea directly this season as they have already played United twice in the Premier League.

Chelsea beat the champions 3-1 at Stamford Bridge last Sunday, a result that left United seventh in the table. The London club could still come up against United in the Champions League but Mata would be ineligible.

With the Premier League fixture list in mind, Wenger is not sure the January transfer window is a fair concept. He said: "Chelsea had already played twice against Man United so they don't play again anymore. They could have sold him last week but it opens up again questions about 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."

It has been suggested that the possible Mata sale could be a move by Chelsea manager, José Mourinho, to damage other title rivals, as United still have to play Arsenal, Manchester City and Liverpool.

But Wenger said: "I wouldn't go as far as that. I just think you would want everybody on the same level. I can understand completely what Chelsea are doing. They do not make the rules, but maybe the rules should be a bit more adapted for more fairness."

Wenger admitted Arsenal will remain on the lookout for "something exceptional" during the final eight days of the January transfer window, but insists there are no imminent deals in the pipeline.

The Gunners are in the market for some additional attacking cover following the season-ending knee injury to England forward Theo Walcott.

However, with Denmark forward Nicklas Bendtner set to return from an ankle injury to the squad for Friday night's FA Cup fourth-round tie against Coventry and Yaya Sanogo also closing in on full match fitness, Wenger does not appear overly concerned about Arsenal's attacking options.