The Stamford Bridge side signed Deco for a reported £8million from Barcelona while Boswingwa arrived from Porto back in May for £16.2million.
Deco had been struggling to get a chance in the first team at the Nou Camp but impressed during Euro 2008 and is Luiz Felipe Scolari’s first signing as Chelsea manager.
Scolari will surely have a few more signings in mind as he settles into his new job and looks to get his hands on some silverware.
Meanwhile, the Gunners have started to strengthen their squad with former United target Aaron Ramsey being their first signing in the summer transfer window. The Reds were one of three clubs to agree a fee with Cardiff City for the young midfielder, who eventually opted to join Arsene Wenger's men.
Ex-Barcelona midfielder Deco will be returning to Old Trafford next season after joining United's title rivals Chelsea in a deal worth a reported £8million.
"Deco is a good player but I can't say I'm that worried or envious of Chelsea buying him."
- Robson7Cantona, Talking Reds
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