The two new signings combined in style against NK Olimpija this afternoon.
Chelsea are taking on NK Olimpija in Slovenia this afternoon, and it has proved a difficult tie so far for the club.
Jose Mourinho's men found themselves 1-0 down at half-time and struggling to get a foothold in the game in a 4-2-3-1 formation.
But at half-time Mourinho tinkered, making a few changes and playing with more of a 4-3-3 formation.
But two players he did not take off the pitch were his ling awaited debutants Cesc Fabregas and Diego Costa. The summer arrivals from Barcelona and Atletico Madrid respectively both got their first taste of action in a Chelsea shirt this afternoon - and they have already shown signs of a fruitful relationship.
Costa scored his first goal for the club after racing clear onto a through ball from, who, you guessed it ... Fabregas.
Fabregas has since left the field, replaced by Oriol Romeu. But Chelsea fans will be hoping to see plenty of this again next season.