West Ham boss David Moyes revealed Kurt Zouma felt sick and was unable to play after withdrawing from the starting line-up in the warm-up before the 2-2 draw against Leicester at King Power Stadium.
Craig Dawson salvaged a point in the first minute of time added on after Jarrod Bowen put the Hammers ahead, but Youri Tielemans' 45th-minute penalty equalised before Ricardo Pereira's header appeared to clinch Leicester's first win in five in all competitions.
Zouma felt sick and substitute Issa Diop started instead with goalkeeper Darren Randolph making up the bench after the defender was pictured on a video on social media kicking his pet cat.
French centre back Zouma has been fined two weeks' wages, around stg£250,000, while two cats have been removed from his property and the RSPCA have launched an investigation.
Moyes said: "He was ill before the game, but he was desperate to play, and we wanted him to play.
"He was sick during the night - he stayed away from the players and didn't eat much.
"He didn't eat with us and he stayed away from the meeting.
"Once he got out there he didn't think it was possible. It is a stomach bug - probably from something he has eaten.
"We told Issa Diop to be ready and he played really well."
Moyes insists his side can compete for a Champions League place after it looked like they were going to lose until Dawson's goal, which came off his upper arm.
"It says lots for the players," he added. "I will squeeze everything out of them and I want us to challenge the top teams."
West Ham seemed unaffected by the Zouma controversy as they took a 10th-minute lead.
Diop lofted the ball forward and Bowen controlled before firing a left-footed drive across Kasper Schmeichel.
Tielemans' penalty levelled on 45 after Aaron Cresswell handled James Maddison's corner.
Pereira met Harvey Barnes's cross with bullet header into the top corner of the net in the 57th minute.
Dawson then got to Bowen's corner with the ball looping in off his upper arm.
Leicester manager Brendan Rodgers felt for his players after they conceded a 14th goal from a set-piece this season, while five of their last seven have come in the last five minutes of matches.
"I really feel for the players because that feels really deflating when we have conceded from a set-piece again," said Rodgers.
"We've had a few of them lately, and it was hard for the players.
"There was anxiety there but possession is a mantra - you have to take the ball and be brave."
Rodgers felt his side were back to their best during the second half.
"In the second half we were outstanding and in the first half West Ham were better," he added.
"The second half was like watching us of old, with the speed of the game and chances. "