Diego Costa, 28, has been frozen out by Chelsea manager Antonio Conte and spent the transfer window angling for a return to former club Atletico that never materialised.
Exiled striker Diego Costa was not included in Chelsea's 25-man squad for the Champions League group phase on Monday.
The win over Middlesbrough left Chelsea seven points clear of Tottenham Hotspur at the summit and means they will be crowned Premier League champions for the second time in three seasons if they win at West Bromwich Albion on Friday.
Costa set up Eden Hazard's early opener, which was cancelled out by Oriol Romeu, before Gary Cahill restored the hosts' lead at an unseasonably cold Stamford Bridge.
Eden Hazard and Diego Costa scored in Premier League leader Chelsea's 2-1 victory over West Ham United on Monday.
Pedro headed in after 65 minutes following an intricate build-up from Chelsea, while Diego Costa scored in the 89th minute to help his side seal a place in the quarter-finals
Diego Costa's 15th goal of the campaign set Chelsea en route to an eighth straight home league win, rubber-stamped by Gary Cahill's late header
Diego Costa headed home Cesar Azpilicueta's cross in the 43rd minute at a foggy Selhurst Park to send Antonio Conte's men nine points clear of second-place Liverpool
Diego Costa's 12th goal of the season saw Chelsea maintain three point advantage over Arsenal. Tottenham Hotspur lost ground after going down 1-0 at Manchester United
Diego Costa's 41st minute winner gave former champions Chelsea a 1-0 win over Middlesbrough on Sunday.
Chelsea are second in the Premier League table after four games with 10 points, two points behind leaders Manchester City. Manchester United are third on nine points
Spain's new coach Julien Lopetegui enjoyed a perfect competitive debut as Diego Costa, David Silva and Alvaro Morata all hit doubles in an 8-0 drubbing of Liechtenstein in Leon
Eden Hazard, via a penalty, and Diego Costa scored for Chelsea while James Collins hit a consolation goal for West Ham at Stamford Bridge.
Besides Diego Costa, World Cup winners Fernando Torres, Juan Mata and Javi Martinez were the other high-profile names left out of Spain's Euro 2016 squad
Diego Costa insisted that he did not bite Gareth Barry after Chelsea were dumped out of the FA Cup by Everton.
Diego Costa was sent off in Chelsea's FA Cup clash against Everton as the team exited the tournament, just days after being dumped out of the Champions League by Paris Saint-Germain.
While Diego Costa hobbled off in the second half of the Chelsea F.C. vs Paris Saint-Germain Champions League last-16 tie, Eden Hazard suffered a hip injury.
Chelsea F.C. are now unbeaten in nine games in all competitions under interim manager Guus Hiddink since the Dutchman took charge following Jose Mourinho's sacking in December.