No the equivalent would be Wenger buying someone, that player being the clubs best player for two seasons, then a new manager turning up and thinking he is the best thing ever and benching said best player because he likes to look in control and powerful.
Mata was chelseas best player last season and the season before. the fans voted him that honour too. he's a regular for spain, improves any side greatly if he plays for them. what happens if mata gets untied top four football and in the summer players decide they want united over chelsea? mourinho is left having sold his best player, strengthened a rival and helped further strengthen them by allowing the mata signing to convince others to go to united.