Jose Mourinho reportedly promised a signed Chelsea shirt to a Hammers ball boy as part of a wager they made before Saturday's game.
The Blues could not break their losing streak, as West Ham emerged victorious with two goals to one at Upton Park, a result which saw Mourinho lose his bet with the ball boy.
Former West Ham player Stuart Slater told the 'Knees Up Mother Brown' podcast that the Portuguese manager had been talking to the ball boys and mascots before the game, when one of them said, "We're going to beat you!"
"When he returned from the warm-up he came back into the tunnel and asked which one of the mascots said West Ham were going to beat Chelsea," said Slater.
"Mourinho told the boy he was going to have a bet with him. He said, 'If we win I'll have your shirt - and if you win, I'll get you a signed Chelsea shirt'."
Slater said it was "brilliant" that Mourinho could engage in a little banter before the game, but said he was not sure if the manager had fulfilled his promise.
Mourinho had other things to worry about on Saturday, after he was sent to the stands for speaking to referee Jon Moss in his room at half-time. Mourinho has been charged with misconduct by the FA, and Chelsea are facing a £25,000 fine for receiving more than five bookings in the Premier League loss.