Pulisic opened the scoring with his first goal since Tuchel took charge in January but, after Thiago Silva was sent off moments later, Chelsea capitulated as West Brom secured an emphatic win at Stamford Bridge.
The United States captain appeared to pick up a knock that forced him to come off at half-time. Pulisic initially entered the pitch alongside his teammates for the start of the second half, only to turn around and speak to his manager before coming 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," Tuchel told reporters after the game.
"So we had to make the decision to substitute him straight away."
The club have not confirmed the extent of his injury but Pulisic is likely to miss Chelsea's Champions League quarterfinal first leg clash against Porto on Tuesday.