Watford manager Roy Hodgson is unsure how long Joshua King will be on the sidelines.
The Norway striker, who has not scored since November, had to be withdrawn during Wednesday's loss to Crystal Palace.
Asked how severe the problem was, Hodgson said: "I don't know, we'll have to see tomorrow,"
"He took a knock in our last training session, unfortunately, so this exacerbated the problem for him.
"But it was the right time to make a change, I thought. Cucho Hernandez has been someone we've had an eye on in training. We of course, Ray and I, don't know him that well. We've not seen that much of him. We've only had a month to work with him in training and we wanted maybe to see what he could do. With Josh perhaps not being 100 per cent in form, it was a good decision, we thought, to take Josh off and give Cucho his chance.
"In the same way we took Tom Cleverley off who'd done well for us but it's the second game in three days. He worked very hard. We thought this was a chance for Edo Kayembe to show what he could do."
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