Jose Mourinho says it was hard to sell Juan Mata to Manchester United, and is "so sorry" he could not keep the midfielder happy at Chelsea.
Mata was named as the club's player of the season in 2012 and 2013, but fell out of favor under Mourinho and completed a 37.1 million pound move to Old Trafford on Saturday.
The Stamford Bridge boss has opted to play Oscar - who scored the only goal of the game in an FA Cup win against Stoke on Sunday - instead, and thinks Mata deserves to play regular first-team soccer.
"These are decisions a manager has to make, but for me it was a hard decision," Mourinho told BBC Radio 5 Live. "I'd love to have him in my squad, absolutely.
"But to have a player in this situation is hard. He deserves that Chelsea open the door for him. I like people to be happy. I'm so sorry I couldn't make him happy in this squad - I'm so sad with that, but I build my team around Oscar playing as my No. 10."
Mourinho expects Mata to receive a warm reception from the Chelsea fans when he eventually returns to Stamford Bridge as a Manchester United player.
He added: "He's very classy. I think he's a player that stays in the fans' hearts and also in those of his colleagues.
"I'm just with him six months and I like him a lot. Imagine the people who have been with him for three years. He stays in the club history - he won important trophies and people loved him. He will come here with a Manchester United shirt on one day and the people will show him that."