Reports in the UK, claim the Blues lodged the bid on Tuesday night which was rejected by United, who maintain that Rooney is not for sale.
Rooney's existence at Old Trafford has been cast into chaos amid claims the player has been left "angry and confused" over comments made by new boss David Moyes which implied that the striker will be second choice again next season to Robin van Persie.
Chelsea & Jose Mourinho's interest has become ever more apparent in the last week or so, claiming his admiration for Rooney, and saying last week that he loved how 'direct' Rooney's style of football is.
United may not be so reluctant to let Rooney go.
Chelsea, though, seem serious in offering Mata - the club's Player of the Year last season - or Luiz, who flourished in a midfield role towards the end of the campaign, a definite sign that they mean business.
Mata's inclusion in the prospective deal has come as a shock to many with the 25-year-old Spaniard a key component of Chelsea's side last season, scoring 20 goals and boasting 28 assists in 64 games.
United, however, are holding still for the moment, but likely would accept an improved cash offer + Mata or Luiz. One feels, as reported this morning on manchesterUnited MAD, that United are angling for a way to get Rooney out of the club on their terms (i.e. Rooney asks for a transfer).
It remains to be seen whether Chelsea will return with an improved offer, although the inclusion of Mata in any deal could yet entice United to do business.
With Shinji Kagawa tipped to take up his favourite No10 position next season behind lone frontman Van Persie, the addition of Mata on the left-hand side would give United an attacking freedom rarely seen since Cristiano Ronaldo's departure in 2009.
With Moyes also keen to replenish United's central midfield options - with both Cesc Fabregas and Marouane Fellaini on the club's radar - the make-up of the Premier League champions' side could be very different come kick-off of the new season on August 17.