The American ace scored the opening goal of a 5-2 defeat to the Baggies but was substituted just after half-time of the clash.
And now Blues boss Thomas Tuchel has revealed the reason behind the decision to take him off.
“Christian did a sprint when he came back out for the second half and said that he felt his hamstring would not survive the game if he continued.
“So we had to make the decision to substitute him straight away.”
Mason Mount replaced Pulisic and also scored but the England international couldn’t stop Chelsea, down to 10-men due to Thiago Silva’s red card, losing heavily to Sam Allardyce’s side.
Chelsea face Porto in the first leg of their Champions League quarter-final tie on Wednesday.
It’s now unlikely Pulisic will feature as the club monitor his fitness.
Tammy Abraham should now come into the squad having missed out against West Brom.
On the striker’s absence on Saturday, Tuchel told talkSPORT: “He was not in the squad. I have to nominate 18 players.
“There are tough choices to make. We take the choices of what the game demands, of what we predict, of the substitutions that we have.
“In the end, we have to select 18 players for more or less 20 positions. So we have to leave three players out and Tammy was one of the three players.
“It’s nothing personal or nothing big. He has the same chance to play every three days like everybody else.”