Slaven Bilic said he took Dimitri Payet off against Manchester United because the West Ham playmaker was showing signs of fatigue.
The Hammers lost 2-0 at the London Stadium in their second match in as many days.
Payet, whose has been linked with a January move, was withdrawn midway through the second half.
But boss Bilic insisted nothing should be read into the substitution other than the fact that Payet, who played the full 90 minutes against Leicester City on Saturday, was tired.
“Dimi is our best player, but we played a couple of days ago and the way he is playing in the game – the position he is playing where he needs to go up and down – it drains you, so his substitution was tactical,” Bilic told reporters after the game.
West Ham faced an uphill task once Sofiane Feghouli was controversially red-carded for a 15th-minute challenge on Phil Jones.
The sending-off was more than harsh, and United took full advantage, with second-half goals from Juan Mata and Zlatan Ibrahimovic giving them the points.
But Bilic was encouraged by his team’s display.
He said: “We are on the rise. We are progressing and in the last few games we have played really good.
“I told the players after the game that if we continue to work and play like this we are are going to be more than good.”