The Spaniard, who had a regular role on the substitutes' bench at Stamford Bridge under Mourinho, moved to Old Trafford for a club-record £37m.
With the Blues needing to score in Wednesday night's Champions League semi-final second leg against Atletico Madrid, Deco believes that Mata would have been a good option.
"I don't know why Jose sold Mata in January because he is a good player," Deco told The Express. "Maybe now he could be important for Chelsea. I am sure Jose knows what he is doing, but Mata seemed to have started well for Chelsea."
The semi-final is currently all square between Chelsea and Atletico following last week's goalless draw at the Vicente Calderon.