The campaign has turned into a slog for Chelsea. In the first half of the season it looked like José Mourinho’s side might canter to the Premier League title but their stride is looking less assured now and the strain of challenging for trophies on three fronts is clear.

The London club still have a five-point lead at the top of the table and a strong chance of ending the season laden with trophies, starting with the Capital One Cup, in which they take on Tottenham Hotspur in Sunday’s final. Victory in that could prove a timely boost that would spill over into the Premier League and Champions League. Defeat, on the other hand, could further deflate a side who were thrillingly buoyant earlier in the campaign.

Chelsea will be without the key midfielder Nemanja Matic for their Wembley showdown after the Serb was sent off for shoving Ashley Barnes in retaliation for a dangerous tackle by the Burnley forward in Saturday’s 1-1 draw. To the fury of Mourinho, Barnes, who, according to Opta statistics, has been the most prolific fouler in the top-flight so far this season, was not punished for the tackle, nor for one earlier on Branislav Ivanovic which Mourinho suggested should also have resulted in a red card. Factoring in two rejected Chelsea penalty appeals, Mourinho claimed the story of this draw boiled down to four bad decisions by the referee Martin Atkinson. But that is not the full story.

Chelsea were indeed hard done by not to be awarded a penalty after Michael Kightly blocked a shot by Ivanovic with his arm, and Barnes’ challenge on Matic could have injured the player, but the other two incidents were open to interpretation, contrary to Mourinho’s declaration. And widening the focus shows that Burnley did more than benefit from refereeing decisions and Chelsea did not do enough to put victory beyond doubt after taking a first-half lead through Ivanovic.

Mourinho devoted his post-match interview to listing the minutes in which the contentious issues arose, but he will no doubt reflect also on statistics that show that Burnley had the same number of shots on target as Chelsea and won the same number of corners.

And, of course, thanks to Ben Mee’s late header from one of those corners, scored the same number of goals. This mirrors a trend which reveals that since Boxing Day, Chelsea’s match average for shots on target, passes and possession have all declined while they have allowed opponents more shots and conceded more goals per game. And, most crucially, the team that had looked so imperious have won only three of their past eight matches.

Performances of some key players have dipped. Diego Costa has not yet regained form following his three-game ban for stamping on Liverpool’s Emre Can last month, and without his sharpness Chelsea struggled to penetrate against Burnley.

Cesc Fàbregas seems similarly rusty as he continues his comeback from injury. The defence is coughing up chances and Mee’s goal proved that Thibaut Courtois cannot always rescue those in front of them. Even Eden Hazard, who spun wonderful magic to create Ivanovic’s goal, lapsed into inconsistency.

Only Ivanovic was constantly excellent, both defensively and going forward, although he could be considered fortunate not to have been disciplined after briefly grasping the referee’s red card in a bid to stop him from showing it to Matic, whose absence for up to three games leaves a void in Chelsea’s midfield.

The post-match verdict of the Chelsea left-back Filipe Luís deserves as much attention as his manager’s barbs. “We have to say Burnley played a good game. They have won points against Manchester City and United, and they are a good team,” said the Brazilian.

“We have to know that every game is not easy, and to be top of the league every game is more difficult because everybody is trying to steal points from us. Now we have to work hard and train more – [and] improve every day because we want to stay top of the league.”

Man of the match Branislav Ivanovic (Chelsea)