Watford coach Walter Mazzarri has doubts over Roberto Pereyra's fitness after the midfielder suffered an injury in Wednesday's defeat to Manchester City.
Pereyra appeared to suffer a knee injury early in the game but tried to play on before eventually leaving the pitch in the 30th minute.
Mazzarri suggested it could be the latest in a series of serious injuries for the Hornets.
"I asked him if he felt a twist in the knee, they [medical staff] told me it was a knock and that's the reason he went back on the pitch. Now, I'm not sure," Mazzarri said.
"We have not been lucky with injuries recently and it's not an easy moment for us. We're not happy with the injury for Pereyra and hopefully he is back soon. We will verify in the next few days."
Mazzarri said his team played well despite ultimately losing 2-0.
"I said it before I think that we played a great game," he said. "It's not easy to play in this stadium against Manchester City against the players of this team so I think we had a great game from our side especially in the second half."
Troy Deeney began the game on the bench and Mazzarri said he wanted him fresh to face Sunderland on Saturday.
"We have had three games in a week and this without Stefano Okaka, the coach said. "Also the fact Deeney had four yellow cards and I wanted to help his condition and be ready for the game against Sunderland.
"We also studied Manchester City and thought that a player of the characteristics of Jerome Sinclair was appropriate for this game."