Joao Ferreira's 53rd-minute equaliser cancelled out Shane Ferguson's close-range opener for Rotherham, but the Hornets are now 13 points adrift of second-placed Sheffield United and a further five behind leaders Burnley.
Bilic said: "I'm disappointed with the result, obviously. The plan was to win, no matter how difficult every game is, especially in the Championship.
"We started well, especially on the right side, and I told the players I can't ask more from them at the moment, but we are missing enough quality in the middle of the park and especially up front.
"The majority of our top players are not on the pitch and we can't rely on the kids to be consistent week in, week out.
"So many times today we were one against one in promising situations facing the opponent and then it's all about quality.
"Up until those top players come back, we have to fight and win as many points as we can."
Bilic at least found a positive in Ferreira's goal, which was struck hard and true into the bottom corner from 20 yards.
"It was a great goal by Ferreira," said the Watford boss. "A great strike and he scored at a good time when there was still enough time for us to win the game and to lift us up, but Rotherham created the best chance after that.
"We didn't convert our situations and didn't end up with the chances where their defenders had to make a foul or something. It wasn't enough.
"You can call it killer instinct, you can call it decision-making, you can call it quality, that's all part of it. Every one of our guys has it, but in those 90 minutes today it was missing. It's clear as sky, simple as that.
"So we're disappointed with the result but every point counts."