(Adds detailS, quotes)
* Deal sooner rather than later says Mourinho
* Essien will be allowed to leave
* Mourinho hits back at Wenger
By Martyn Herman
LONDON, Jan 24 (Reuters) - Jose Mourinho said he was sad to see midfielder Juan Mata leaving Stamford Bridge after confirming that the Spaniard's move to Manchester United was imminent on Friday.
United are expected to complete a club record deal, reported to be 37 million pounds ($61.5 million), for playmaker Mata who has found first-team opportunities hard to come by since Mourinho's return.
"It is a good offer. Respectful to us, respectful to the value of the player. That allows to try to bring another players because we need another player," Mourinho told a news conference. "Sooner rather than later things will be finalised.
"I asked him when the offer comes are you really happy to go and he was saying yes I am really happy to go. After that we could feel mixed emotions.
"We are happy he goes to fantastic situation but we are all sad as well and some of him was a bit the same."
"No one is jumping about because Juan is leaving. We all like him a lot and we can't say you can only go if you go to Russia, to China, to France or Portugal.
"I don't know if Chelsea supporters globally would be happy with that because we are an open club."
Since joining from Valencia for 23.5 million pounds in August 2011 Mata has twice been voted Player of the Year at Stamford Bridge but has slipped below Eden Hazard, Willian and Oscar in the midfield pecking order this season.
"It's a good move for Juan, it's good for the Premier League and the offer was right. We hope Juan does well. We are convinced he will do well. No fears for us."
Mourinho said he was confident the deal to sign Basel's Egyptian playmaker Mohamed Salah would be finalised but said the club would not be buying another striker in the transfer window despite Fernando Torres being injured.
He also said Ghana midfielder Michael Essien "could go or stay" in January.
"Essien is at this moment in a position where he can stay or go. If he stays fantastic. He is a fantastic club man, team player, so if he stays it's fantastic.
"But if a great situation arrives for him, for his future, in relation to him going to the World Cup, or a few more years contract at a high level, we must show him the respect."
Chelsea being prepared to sell Mata, twice their Player of the Season and a huge fans' favourite, to rivals United has raised eyebrows and even led Arsenal manager Arsene Wenger to suggest Chelsea are "playing" with the transfer window.
Wenger's point was that Chelsea have already played United twice this season so will not have to face Mata in a red shirt.
Predictably, Mourinho dismissed Wenger's claims and launched an attack against what he thinks is Arsenal's preferential treatment in the fixture list.
"Wenger complaining is normal because that's what he does all the time," Mourinho said.
"But when a player plays for a club on August 20 and two weeks later plays against them with a different shirt this is normal because this is the market, we don't make the rules.
"If Wenger sold Mesut Ozil to Manchester United in this moment I would be very happy because he is selling a very important player. He should be very happy that Chelsea sell a player like Juan Mata but I think this is his nature."
Arsenal play Coventry City later on Friday in the FA Cup whereas Chelsea are in action on Sunday against Stoke City. Both are in midweek action in the Premier League next week.
"We have to accept the way he is but when he says this is not fair I think what is not fair is that his team always has the best days to play," Mourinho said.
"In the League Cup we played 24 hours after, they had 72 hours. This week one plays Friday and one plays Sunday, who plays Friday and who plays Sunday?
"This is not fair because they always get the right time to rest and the right time to play, it's fantastic." ($1 = 0.6013 British pounds) (Reporting by Martyn Herman, editing by Ed Osmond)