Of course, the Blues still sit four points clear at the top having played a game more than Liverpool and Arsenal, but suddenly the Gunners have been given great encouragement ahead of their trip to Stamford Bridge on Saturday. After they lost to Aston Villa - who have been dreadful for most of the season - Chelsea's trips to Crystal Palace and Swansea in the next four league outings look a whole lot trickier.
The simple truth is that they never got going against Villa. With Fernando Torres lining up in attack, the Blues lacked a focal point in the final third to link with Eden Hazard in the role Samuel Eto'o normally performs. Torres failed to hit the target with any of his five attempts during his 67 minutes on the pitch - once again reinforcing the evidence of his sorry decline.
While Mourinho must know to expect little from Torres by this stage, he will be more concerned by Oscar's drop in form. So disappointed was the manager with the Brazilian's display on Saturday that he took the risk of replacing him ahead of Willian, who was already on a yellow card. Two minutes later, Willian was off, as Chelsea's task became even more difficult.
Mourinho didn't help matters by his exaggerated remonstration on the touchline that surely fed into the mindset of his players. Chelsea completely lost their cool, lacking the composure to withstand Villa's late pressure and create chances of their own to secure a vital result. As the tension increased, it was no surprise to see Ramires crack as he almost did the same to Karim El Ahmadi's shin.
With the Brazilian duo now missing for the Arsenal clash, Mourinho will be forced into deviating from his usual plan in the tougher tests. Chelsea will not be able to counter at the same speed or press with the same intensity as they did at home to Manchester United and away at Manchester City. Preparing for this change will not be easy, either, with Galatasaray visiting Stamford Bridge in midweek.
Liverpool and Brendan Rodgers
A truly dominant display founded on unfamiliar defensive resolve and quick, counter-attacking football that Manchester United simply could not cope with. Liverpool played as though we expected, yet the champions looked woefully unprepared for the biggest game of their season so far.
The truth is that Liverpool did not even have to play at their brilliant best to secure a 3-0 victory that underlines their status as title challengers. Daniel Sturridge fluffed his lines twice in the first half, Luis Suarez's footwork was not quite as nimble as usual, and Raheem Sterling struggled to have the same impact through the centre. But, as many teams have found at Old Trafford this season, it mattered little.
The Reds have now recorded a 43-point swing over their rivals from this stage last year; the gap to Chelsea is only four points with a game in hand at home to Sunderland on March 26; the lead over United is now 14 points and 29 goals - on the pitch on Sunday, it looked as though it could have been even more. Liverpool played like contenders; the champions, a fallen giant.
Brendan Rodgers will have been particularly pleased with the platform his defence provided for an impressive victory. Martin Skrtel was excellent, bossing the back-line in the way Nemanja Vidic used to do for United. Glen Johnson and Jon Flanagan worked tirelessly to snuff out the hosts' predictable plan to attack from the flanks and then burst forward in support when Liverpool had the chance to break - an approach that was aided by Simon Mignolet's careful distribution.
It may not have been their most inventive performance of the campaign, but Liverpool were organised and clever in their use of the ball. They looked like a cohesive unit - every man understanding his specific role - while United were reminiscent of Tottenham in recent weeks - mindful of showing desire and commitment, but clueless as to how to apply those qualities in a way that would eke out an important victory.
While David Moyes should not have taken United from Premier League champions to seventh inside eight months, Rodgers' transformation of Liverpool has been equally surprising. He has been repeatedly mocked him for his soundbites but this doesn't detract from Rodgers' status as one of the best managers in the game - and he is still only 41.
That none of the Reds' starting XI on Sunday have been signed in the last two transfer windows points to Rodgers' strength as a coach who can develop his players. With money to spend in the market, he has not always convinced. On the training ground, he has taken a team that finished eighth in 2011/12 with just 52 points and 47 goals to second with 62 points and 76 goals after just 29 games. It is a phenomenal achievement.
"We're probably a year ahead of where we've been but that's testament to the players and how quickly they're adapting to the methods," said the manager as he played down Liverpool's title chances in the wake of their most significant victory to date. Considering the Reds' form and fixture list - with Chelsea and Manchester City to come at home - the opportunity is certainly there.
A hard-fought victory in the north London derby despite Arsene Wenger's annual battle with a predictably depleted squad. Arsenal showed great spirit to dig in against Spurs to move level on points with Liverpool and stoke the embers of their title challenge, but it is still difficult to believe they can win the league this season. Not that I wish to spoil anyone's fun.
Against Chelsea on Saturday, the Gunners will need to be more ruthless. They have won only four of their last 25 matches against the Blues and failed to score in both of their previous clashes with Jose Mourinho's side this season. Alex Oxlade-Chamberlain cannot afford to miss the sort of chances he passed up at White Hart Lane if Arsenal are to further close the gap and really make a fight of it at the top.
While there are no concerns about the defence - with Laurent Koscielny and Per Mertesacker impressing again on Sunday - Arsenal's worries lie in a lack of attacking options that Wenger failed to address last summer or in January. The manager will hope that Aaron Ramsey brings back his goal threat when he soon returns from injury but, without Mesut Ozil and Theo Walcott, Arsenal will struggle to stretch opponents of Chelsea, Man City and Everton's quality.
Lukas Podolski is clearly not the answer, while Olivier Giroud - who has flattered to deceive this year - didn't manage a single shot in 90 minutes against Spurs. The telling statistic that the striker has scored just once in 15 matches (in all competitions) against Chelsea, Liverpool, Man City and Man United suggests that he is not going to be the difference at Stamford Bridge or against City at the Emirates.
Arsenal will no doubt battle until the end now they are fighting on only two fronts, but there is a sense that they are being kept at arm's length in the title race. Despite being level with Liverpool, it will take an enormous effort to overcome an injury crisis that Wenger should have foreseen.