Allardyce’s men ended Chelsea manager, Thomas Tuchel’s 14-match unbeaten run, thanks to goals from Matheus Pereira (2), Callum Robinson (2) and Mbaye Diagne at Stamford Bridge on Saturday.
But Allardyce believes Mendy was at fault for Pereira’s first goal and feels the midfielder’s outfit did well to pressure the Senegalese shot-stopper during the encounter.
According to him, Mendy’s rashness cost Chelsea to concede the first goal, adding that the goalkeeper came dashing to the edge of his box and didn’t get anywhere near the ball.
“That is why we played two splits upfront with Matheus Pereira and Mbaye,” Allardyce told TalkSport.
“The press worked very, very well, and there is something with their goalkeeper [Edouard Mendy] who can be rash.
“He can rush out. If you saw the first goal we scored, he came dashing to the edge of his box and didn’t get anywhere near.
“So we felt every time it went back to the goalie we would put him under pressure as well.
“Of course, that gave us good opportunities and then obviously you could say the sending off helped, playing against ten men, but you have still got to beat what is a very good side.”