Rafael Benitez's difficult start as interim Chelsea manager took a significant turn for the worse when his side were beaten 3-1 by West Ham United at Upton Park on Saturday.
It left Chelsea without a win in seven Premier League games - their worst league run since February 1995. Benitez has been in charge of three of those games, but after successive scoreless draws, the manager was entitled to feel confident he would oversee his first victory when Mata's goal capped a dominant first-half display.
But West Ham were transformed after manager Sam Allardyce introduced Diame and Matt Taylor at half time and ultimately deserved victory for a vastly improved performance in the second period
After the frustrations of Benitez's first few days in charge at Stamford Bridge, Chelsea desperately wanted to make a positive start. After surviving an early lapse when Winston Reid was allowed a free header in the Blues penalty area, they opened up the West Ham defence with ease to allow Mata to score the confidence-lifting opening goal.
The move was as simple as it was incisive, stemming from a throw-in on the half-way line.
Moses released Torres on the right-hand side of the West Ham area and the striker's cut-back found Mata, who finished side-footed from 12 yards.
The effect on Chelsea was tangible and Benitez's side began to press forward with a fluency that was noticeably absent during their previous two games.
Ramires found space on the left to set up Moses, who shot wide, before a sweeping four-man break ended with Torres firing wastefully over.
West Ham had Jussi Jaaskelainen to thank for restricting Chelsea to a single-goal lead at half-time after the goalkeeper produced an excellent save to repel Mata's close-range shot.
Allardyce made two changes at the break, introducing Diame and Taylor for James Tomkins and Gary O'Neil.
And the changes had an immediate effect, with the Hammers transformed from the side that had struggled in the first half. Immediately, Chelsea were pressed onto the back foot and forced to defend with increasing desperation. But their back-line was broken when Cole out-jumped Branislav Ivanovic to head the home side level in the 63rd minute.
Chelsea responded positively, Mata hitting the post with a curling free-kick.
But the momentum was with West Ham and Allardyce's side went ahead in the 86th minute when Diame latched onto Cole's lay-off to drill the ball home. Substitute Maiga sealed the points when he found the roof of the net after Taylor's shot had been parried by Cech.