It left Chelsea without a win in seven Premier League games - their worst 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.
Benitez made four changes to the side that started the 0-0 draw against Fulham, underlining the manager's pre-match assertion that the demands of the season are already beginning to tell on Chelsea's senior players.
Agger boosts Reds Daniel Agger's first goal of the season earned stuttering Liverpool a 1-0 win at home to promoted Southampton on Saturday, giving the Reds just their second Premier League victory in seven matches.
Agger struck three minutes before half-time to clinch only a fourth win of the campaign for Brendan Rodgers' men, as the Reds bounced back from Wednesday's 2-1 reverse at Tottenham. Liverpool, whose eight-match unbeaten run in the league was snapped following that midweek defeat, climbed one spot to 11th in the table, while Southampton remained mired in the relegation zone after their first loss in five outings. Lucas Leiva's return to the Liverpool midfield was a welcome sight following the Brazilian's three-month lay-off with a thigh injury, and the home side went close to taking the lead inside 10 minutes as Glen Johnson's close-range effort travelled just wide.