The Reds looked on course for an opening-day victory away from before Miguel Britos bundled the ball over the line in the 94th minute to hand his side a 3-3 draw.
Replays showed he was clearly standing in an offside position in front of keeper Simon Mignolet when Richarlison put the ball across goal, however.
“It’s obvious because the linesman is on the line. He needs to see it,” said Klopp after the match.
“I cannot change it. We would have had defensive areas we need to work on if we’d won 3-2. We were the better team, we should have won.”
Klopp said his side were much better in the second half, but didn’t close the game.
“The first half I wasn’t too happy with. We passed but couldn’t see what we actually wanted,” he said.
“The second half was a lot better. We forgot to close the game. We defended a little deep. We have to push up. Both teams struggled with 90 minutes, it’s the first game. That’s normal.”
Watford boss Marco Silva said he had not seen whether his side’s equaliser was offside.
“I think it’s a fair result. We had a very good first half, a good intensity like I wanted,” said the former Hull boss.
“I didn’t like the first 25 minutes of the second half. We expected a good reaction from Liverpool but we needed to be better. We made mistakes. But we reacted well.
“Our fans pushed our players on. We had some small injuries. We need to show this attitude and commitment in all our games.”