Lascelles attempted to run off a knee injury when running out for the second half against Chelsea - but complained of a new issue within seconds before being hooked.
The skipper had picked up a knock in the first half in the 2-0 defeat against the Blues and Fabian Schar had been told to warm-up at the break as it became likely Lascelles was struggling.
But Lascelles appeared to change his mind after emerging from the tunnel for the second half before Schar, who was ready after a rigorous warm-up, replaced him.
Bruce said: "Unfortunately he wanted to have a go with his knee, he hurt it.
"The reason he came off is not his knee, when he went to run out he felt his thigh.
"It's a strange and unique situation.
"We don't know how bad it is until we see him on Monday."
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