The Spain international struggled to make anything happen during the game before he scored the winning goal, sparking wild celebrations from his team-mates.
Chelsea returned to the top of the table with their ninth successive Premier League win as they beat West Brom 1-0 at Stamford Bridge.
Arsenal had temporarily toppled the Blues with victory over Stoke City on Saturday, but Antonio Conte’s side reclaimed pole position, Diego Costa finally breaking through a dogged West Brom defence for the decisive goal, which marked his 12th of the season.
Tony Pulis’ men went into the game on a four-match unbeaten run but leave west London without reward despite a promising first half in which Salomon Rondon had the best chance.
Chelsea offered little after the restart to suggest they would break the deadlock, with Costa largely anonymous as Willian and Cesc Fabregas were introduced from the bench.
However, after 76 minutes, Costa muscled his way through on the right and blasted a stunning left-footed finish into the top-left corner to settle the contest.
West Brom, who would have been good value for a point, could not hit back, allowing Chelsea to re-establish a three-point cushion at the summit.
The visitors started brightly, with Chris Brunt lashing their first notable effort wide from 20 yards, before Rondon stole possession from David Luiz on the touchline and drove a shot narrowly off target.
Chelsea responded as Pedro directed N’Golo Kante’s speculative strike just past the right-hand post, and Costa could not stretch to turn a teasing left-wing cross into the net.
David Luiz dipped a free-kick towards goal shortly before the break, but Ben Foster was down to save comfortably.
Although Foster was sent sprawling as another David Luiz set-piece deflected wide after the restart, Chelsea still could not build momentum.
Matt Phillips took possession from Willian and sought out Rondon in the middle, but Gary Cahill was on hand to make an important block, and produced a similar dive in front of the onrushing Darren Fletcher as a loose ball fell to the midfielder on the edge of the box.
Nemanja Matic dragged an ambitious long-range effort wide, with Willian following suit with a tame effort from the right.
But, with 14 minutes remaining, Costa came up with the goods.
Bustling past Gareth McAuley, Costa created a shooting angle and drilled a thunderous effort past the helpless Foster to finally open the scoring.
West Brom tried to muster a response, but could not forge a serious opening in the final minutes as Chelsea comfortably held firm, Marcos Alonso almost doubling the hosts’ lead late on.