Battling West Brom kept their slim survival hopes alive with a deserved 3-0 win over Southampton.
The second-bottom Baggies closed the gap to Premier League safety to eight points with seven games left.
Matheus Pereira’s penalty and goals from Matt Phillips and Callum Robinson gave Albion their biggest league win of the season.
Buoyed by that win at Stamford Bridge, confident Albion enjoyed one of their best performances of the season on Monday.
They were controversially denied a fourth-minute opener after a VAR quirk, which only adds to the argument against the technology.
Pereira’s free-kick found Phillips and Fraser Forster parried the winger’s effort.
Southampton only managed to clear the ball to Darnell Furlong on the edge of the area and his shot was turned in by Diagne – only for the forward to be swiftly ruled offside.
The decision was backed up after a lengthy VAR check, yet Diagne appeared to be onside.
But VAR official Kevin Friend stuck with the on-field decision because those at Stockley Park could not prove whether Diagne’s was on or offside.
Albion refused to be disheartened and after Sam Johnstone beat Kyle Walker-Peters’ drive behind, they continued to threaten with Furlong heading wide and Pereira nodding at Forster.
While Albion endured some panicked moments at the back, no less than when Danny Ings curled wide and then sent the ball across the face of goal, they remained in command.
Diagne, though, should have fired them ahead instead of slicing well wide with just Forster to beat after 28 minutes.
Stuart Armstrong had Johnstone at full stretch soon after but the Baggies did not have to wait long for two goals to arrive in three minutes.
They opened the scoring from the spot after 32 minutes when a slick move involving Diagne and Ainsley Maitland-Niles ended when Forster clattered Pereira, and the forward got up to drill the penalty down the middle.
It was quickly 2-0 when Phillips converted Diagne’s cross at the far post to reflect Albion’s first-half efforts.
Faced with another mountain to climb, Southampton failed to recover despite improving after the break.
Armstrong shot at Johnstone after an hour but they could not break the Baggies down, with the hosts having found their resilience under Allardyce.
Albion even added a third with 21 minutes left when Okay Yoksulu’s ball found Robinson and he held off Jannik Vestergaard to fire past Forster.
Southampton did have the chance for an injury-time consolation when Conor Townsend fouled Moussa Djenepo in the box but Johnstone saved Ward-Prowse’s spot kick.
