Oscar's fellow Brazilian Willian made a slow start to the campaign after spurning a move to Tottenham Hotspur at the last minute in favour of a big-money transfer to Stamford Bridge from Russians Anzhi Makhachkala in August.
Eden Hazard and Oscar have always been first choices under manager Mourinho in the formation of three players that operate just behind the sole striker in the London club's 4-2-3-1 system.
In the last few weeks, however, the fleet-footed Willian has brought a dynamism to the attacking midfield that has had Mourinho drooling and helped lift Chelsea into third place in the table, two points behind leaders Arsenal.
The manager gives Willian, Oscar and Hazard a licence to thrill in possession but he also insists on a tigerish defensive approach from the interchangeable creative trio when the team lose the ball.
It has taken the three players a while to adjust to Mourinho's demands but a run of six wins in a row in all competitions is tangible proof the approach is working.
"It is true that most of the time when I was at Shakhtar Donetsk I played out on the wings but I also played No. 10 from time to time there," the 25-year-old Willian told the Chelsea website (www.chelseafc.com) earlier this month.
"It is similar here, sometimes I am out on the right, sometimes out on the left, but also playing the No. 10 position.
"The No. 10 needs to have the quality to find space and make the right runs in the right positions. You have less space and there are more people around you when you play there but the knowledge I have from playing wide does help," said the mop-haired Willian.
"I am still settling into this particular position but I don't see any problems with how I am playing at the moment."
Mata is the darling of the Stamford Bridge crowd, having been voted Player of the Year when Chelsea won the Champions League in season 2011-12 and again last term when they lifted the Europa League under Mourinho's predecessor Rafa Benitez.
No one doubts the diminutive Spaniard's silky skills and cunning on the ball but it seems that in Mourinho's eyes he does not have the necessary pace or tackling ability to provide the defensive barrier he craves from his attacking trio.
British newspapers were awash with speculation on Tuesday that Mata may be heading to league champions Manchester United, the side Chelsea beat 3-1 at Stamford Bridge on Sunday, for a fee in the region of £37 million.
The Spanish international has been ignored by Mourinho since he showed visible frustration at being substituted early in the second half of the 3-0 league win at Southampton on New Year's Day.
Mata has been in and out of the team all season and had always reacted in a calm and dignified manner until events at Southampton got the better of him.
He threw his arms in the air and slammed the back of the seat in front of him with his hand after being hauled off.
If any other Chelsea manager had treated Mata the way Mourinho has this season there would have been an outcry among the supporters.
The 'Special One', though, has the unconditional backing of the fans after returning in the close season for a second stint in charge following a highly successful first spell that led to league title triumphs in 2005 and 2006.