Scott Parker’s wait for a first victory as West Ham United manager continued as Blackburn Rovers claimed a 2-1 win at Ewood Park.
Second-half finishes from Andri Gudjohnsen and Dapo Afolayan put Blackburn in control before Joel Piroe’s late reply gave the visitors hope.
However, West Ham could not find an equaliser and dropped out of the Championship play-off places after a third defeat in four matches.
Parker’s opening game in charge had ended in a dramatic 3-3 draw at Cardiff City on Wednesday, with the Hammers recovering from three goals down after he replaced Nuno Espirito Santo.
Despite the support of 7,000 travelling fans, they were unable to produce another comeback in Lancashire.
West Ham enjoyed a promising spell after half-time, with captain Jarrod Bowen striking the woodwork as the visitors pressed for the opening goal.
Instead, Blackburn broke through on the counter-attack.
Jayden Fevrier surged forward and delivered a cross for Gudjohnsen to head home.
Afolayan then doubled the advantage, beating Mads Hermansen while West Ham appealed for play to be stopped as Max Kilman lay on the turf holding his head.
Piroe reduced the deficit inside the final 10 minutes, ending a run of eight games without a goal, but Blackburn held firm.
Tomas Soucek had a late opportunity to rescue a point, only to direct his header straight at goalkeeper Balazs Toth.
Victory ended Blackburn’s run of successive defeats and lifted Tony Mowbray’s side to 13th, just two points behind West Ham.
They return from the international break with a derby trip to Burnley.
Defensive problems leave Parker with work to do
Parker arrived at Ewood Park having won on each of his previous three visits as a manager during promotion campaigns with Fulham, Bournemouth and Burnley.
Repeating that success with West Ham will require him to address a defence that continues to undermine their promotion ambitions.
The Hammers showed encouraging signs either side of the interval.
Divine Mukasa tested Toth before the break, while Konstantinos Mavropanos headed narrowly wide early in the second half.
Bowen and Arne Engels also threatened as the visitors built momentum.
Yet their vulnerability was exposed once again when Blackburn turned defence into attack to take the lead.
West Ham have now conceded 23 league goals, comfortably the highest total among the Championship’s top 10, with 11 coming in their past four matches.
That record contrasts sharply with Parker’s Burnley side, who allowed just 16 goals throughout their promotion-winning campaign two seasons ago.
His first two matches have highlighted both West Ham’s attacking potential and their defensive frailties.
With just over two-thirds of the season remaining, finding greater consistency will be essential to their hopes of an immediate Premier League return.
Blackburn Rovers head coach Tony Mowbray told BBC Radio Lancashire:
"We all know how emotional football is and generally when you lose a game, people usually see the negative side of a performance or the work rate or whatever it is, but I've been pretty happy with this group of players since the day I walked in.
"They care about each other, they work hard for each other, so we're delighted today. That's a tough game. You can feel their quality, their power and strength.
"We scored at good times. We always carried a threat whilst we allowed them a fair bit of the ball. It's not wise to go and chase them and hunt down really good football teams who can pick the right pass so we were pretty disciplined.
"After the back of the results, the work rate of the team, the endeavour and the commitment in such a big game, it was positive for us. Our lads rose to the challenge against a very good team."
West Ham United head coach Scott Parker told BBC Radio London:
"It's very disappointing. We dominated first half without creating too much and we suffocated Blackburn for large parts.
"Second half, we came out really sharp and really bright but we didn't manage to score and their first goal gave us a bit of a hill to climb.
"We proved in the week we can do that but we weren't clinical enough in our moments. We probed really and then the second goal is obviously a foul, but we gave ourselves a mountain to climb."
