Jurgen Klopp rued Liverpool missing out on a "clear penalty" for Marouane Fellaini's challenge on Sadio Mane in the closing stages of their 2-1 defeat at Manchester United on Saturday.
Substitute Fellaini appeared to clip Mane inside the box in the 82nd minute, but referee Craig Pawson signalled for play to continued.
United held on for a win that sent them five points clear of third-placed Liverpool in the Premier League, although Tottenham can move to within four by beating Bournemouth on Sunday, leaving Klopp frustrated.
"It was a clear penalty with Fellaini on Sadio, in situations like this you need the right decision. We didn't get them so we lost 2-1," he told Sky Sports.
Marcus Rashford scored United's goals after Romelu Lukaku overpowered Dejan Lovren to bring down long balls up the pitch, and Klopp felt his team should have been more resilient at the back.
"We have to defend these situations better. You can always lose a header or a challenge with Lukaku of course but there is a gap in behind and we need to close that," he said.
"The two goals made the game not easy. We had a lot of OK moments, two good moments in the first half where we could have finished a little better, more clinical. We didn't score before half-time.
"In the second half we chased the game which against a team like Manchester United is really difficult.
"It is all about the result and we lost."
On why Liverpool weren't at their best, he added: "I have no explanation for that because it's clear. They played a lot of long balls and we were not there.
"The second ball was one thing but I think on the first for Rashford, when he cuts back we need to be there. We were not at 100 per cent."
