Walter Mazzarri believes Mauro Zarate's injury cost Watford in the second half of their 1-1 draw at home to West Ham.
Zarate won the penalty from which Troy Deeney handed Watford the lead and he was influential before suffering an apparently serious injury shortly before half-time.
West Ham came back into the game in the second half, equalising through Andre Ayew and pushing for a winner before Michail Antonio was sent off.
And Mazzarri pointed to Zarate's withdrawal as a key point in the game, suggesting Watford missed the 29-year-old, while his enforced withdrawal also limited the manager's options.
"I saw Zarate in a lot of pain," Mazzarri told BBC Sport. "Unfortunately it is not a good sign.
"He was also making a lot of difference on the pitch. Hopefully it is not too bad. But, if we speak about the game, I didn't like how we went down in the second half.
"We also committed some silly mistakes that we can avoid. It's not acceptable to allow them a goal on the counter-attack when you are winning 1-0.
"The injury also didn't allow me to make all the substitutions I wanted."
Slaven Bilic believes it was two points dropped for his West Ham side, despite fighting back from a goal down.
"Considering we were in good form and how well we played today, especially in the second half, it's two points dropped," he said.
"On the other hand, considering we are playing away against a good team and we conceded an early goal, there's always a danger you will concede a second then you can lose the game.
"Yeah, we lost two points - that's my point of view - but it's a good point. It's hard to come back away."