Chelsea missed the chance to ensure they would stay ahead of Barclays Premier League title rivals Arsenal at the end of the midweek fixtures as Victor Anichebe's late goal saw West Brom hold them to a 1-1 draw.
Chelsea were poised to move four points clear at the top of the table on the back of defender Branislav Ivanovic's injury-time strike in the first half at The Hawthorns.
But substitute Anichebe flicked home his third goal of the season in the 87th minute to make it 1-1 and thwart the Blues, who had lost on their last two visits to this part of the West Midlands.
After all the hype in the wake of their stunning win at Manchester City and Eden Hazard-inspired thumping of Newcastle on Saturday, Mourinho's side were undoubtedly off the boil against the Baggies.
Perhaps Mourinho does have a point after all, that his side are the "little horses" in this year's championship race, as Arsenal and City will have the chance to take over at the summit on Wednesday.
Only Willian can claim to have had an impressive game as it was he who primarily pulled the strings, and who crucially set up the only goal of the first half just as Albion were on the brink of heading into the interval on the back of a creditable 0-0 scoreline.
From Willian's corner into the near post, centre-back David Luiz was left unmarked to flick the ball on with his right foot to the far post where Ivanovic slotted home from three yards.
After the break Willian enjoyed a further three driven efforts, all of which were superbly saved by home goalkeeper Ben Foster.
At least the home side showed greater attacking intent in the second period as James Morrison fired wide, Thievy Bifouma flashed a shot through the six-yard box, while skipper Chris Brunt hammered a half -volley beyond the left-hand post.
With Anichebe on for Bifouma in the 73rd minute, the Nigerian glanced home Saido Berahino's cross three minutes from time to lift Albion back out of the relegation zone.
Come the conclusion it was a fair result, and all credit to Albion who pressed and harried admirably throughout the entire game, even if they failed to record a single shot on target in the first 45 minutes.
It has been the desire of new head coach Pepe Mel to see his players give the opposition not a minute's respite, and Chelsea were rarely allowed any time on the ball at any stage.
That was underlined by the minimal number of chances that came their way during the opening half prior to Ivanovic's injury-time effort.
The key ones fell Willian's way as he first saw a fierce 27th-minute drive flash just over the angle of bar and post.
Three minutes from the break he picked up a loose ball on the edge of the area before chesting it down and sending a half-volley dipping over the goal.
Then came Ivanovic's goal - a blow for Albion, although they had conjured just one chance of note in the first period.
That came as Morgan Amalfitano, Berahino and Bifouma - making his first start since his recent loan move from Espanyol - combined to tee up Brunt for a searing shot two yards wide.
The first half was most notable for the boos that rang in the ears of Ramires - playing in place of Frank Lampard in the only change to the Chelsea team that beat Newcastle.
Ramires was the player who contentiously earned Chelsea a penalty in the 2-2 draw at Stamford Bridge in October, when Albion were seconds away from a shock victory.
The Albion fans certainly had not forgotten that moment as Ramires' every touch was greeted with jeers and catcalls.
At one point he went down under a challenge from Bifouma, which left him requiring treatment, and resulted in even louder boos when he returned to the pitch.
After the break Willian came the closest to extending Chelsea's lead, but found Foster in fine form.
As the half wore on the home side finally found some impetus up front and began to create their opportunities prior to Anichebe coming up trumps to send the home fans into raptures.
Just nine days ago, Anichebe had again come off the bench to salvage a point for Albion in a 1-1 home draw with Liverpool.