José Mourinho has hit back at Arsène Wenger over the sale of Juan Mata to Manchester United, saying: "Wenger complaining is normal because he always does."

The Arsenal manager had suggested on Thursday that Chelsea selling the Spanish playmaker to United, having played the champions twice whereas the other title contenders have still to face David Moyes's side, would give Chelsea an unfair advantage.

Mourinho, who on Friday confirmed that Mata is on his way to United and will have a medical before his £40m move there, added that Arsenal seem to have an advantage when it comes to the fixture list. "Wenger complaining is normal, because he always does. It's something that we know. When a player plays for a club on August 20 and two weeks later on September 5 plays against the same club with a different shirt, this is normal, because this is the market.

"We don't do the market, we don't do the rules, we have just to do according to the rules. If Wenger sells [Mesut] Özil to Man United in this moment I would be very happy because he sells a very important player. Normally he should be happy that Chelsea sold a player like Juan Mata, but this is a little bit his nature. When he says this is not fair, I think what is not fair is that his team always has the best days to play [getting more rest inbetween games]."

Mourinho added that the Mata transfer was completely separate to any potential deal for Wayne Rooney to go in the opposite direction: "If we said to United: 'You didn't sell us Wayne Rooney in the summer so we won't sell you Juan Mata,' that is not right."

He added: "We had an offer that is a right offer. I asked him [Mata]: 'Are you really happy to go?' He said: 'Yes, I'm really happy to go.'"

Mata was Chelsea's player of the year for the last two seasons but has been marginalised at the London club this campaign following the return of Mourinho as manager in the summer. Mata has largely been on the bench, with Mourinho preferring Eden Hazard, Willian and Oscar as his playmakers.

"He was fantastic for Chelsea. Chelsea was fantastic for him," Mourinho said. "We want him to go there and be happy and do well for Manchester United." While United has been quiet on Mata's likely transfer on Friday, Mourinho said: "Things are going in a good direction."

José Mourinho tries to get a point across to Juan Mata in August. The Spaniard is set to leave Chelsea for Manchester United. Photograph: Ian Kington/AFP/Getty Images

The Manchester City manager, Manuel Pellegrini, added his views on the transfer on Friday, saying he does not believe transfers such as Mata's impending switch should be allowed mid-season.

Pellegrini has no issue with United strengthening their squad during this month's transfer window – as they are entitled to do – but he is against the principle of big-money moves during the campaign.

The Chilean said: "I think Juan Mata is a very good player. It is a shame he didn't play often this year at Chelsea. I think United are buying a very good player.

"Personally I don't agree that the player can go from one team to another team in the same league at this part of the year, but rules are rules and he can change clubs."

Chelsea, meanwhile, moved quickly to replace Mata with Mohamed Salah after reaching an agreement with Basel, subject to personal terms and a medical. "It was a surprise for us that Juan was leaving," Mourinho said. "We were not prepared to make a bid or to try to bring a player to replace Juan in the squad so everything happened very, very fast.

"We decided to try to go for Salah. We started from zero; no contacts with the club, no contacts with the player. The first step was an agreement with the club. That situation is done, no more discussions, everything is signed and done subject to the medical, that we didn't have yet and subject to the player's desire to join us or not. Now we know he wants [to come]."

Mourinho has spoken to Salah, who was also interesting Liverpool. Brendan Rodgers has not given up hope and Mourinho admits a last-minute U-turn is feasible, although the Egypt winger's preferred destination is west London.

Mourinho said: "We know that the player wants to come. He told me personally. Now it's [personal] terms, the medical that he has to pass probably in Zurich or Lausanne and hopefully he comes. Probably the situation is still open and not closed."

Salah is set to become Chelsea's third signing of the winter transfer window, following the captures of Bertrand Traoré and Nemanja Matic. Kevin de Bruyne has already left for Wolfsburg, Mata is en route to Manchester and Michael Essien has permission to leave, too.

"Michael Essien is in this moment in a position where he can go or he can stay," Mourinho said. "If he stays we are more than happy for him to stay, but he's another player that gave everything to this club and if a great situation for him arrives for his future … we must show him respect. If he stays, he stays and if he stays he's playing against Stoke."

Chelsea play Stoke on Sunday in the FA Cup fourth round, with striker Fernando Torres (knee) missing for three weeks.