Liverpool searched for an equalizer in the second half, Wijnaldum had a shot blocked by the hand of Jack Stephens, while forward Mane went down under a challenge from Walker-Peters.
“[The handball] looked like a clear penalty,” Klopp said. “I turned to the fourth official, he said: ‘We checked already, no penalty.’
“What [referee] Andre Marriner did with Sadio Mane tonight, I’m not sure that’s OK, to be honest.”
Klopp added: “I hear now that Manchester United had more penalties in two years than I had in five and a half years. I’ve no idea if that’s my fault, or how that can happen.
“But it’s no excuse for the performance. We cannot change it, we have to respect the decisions. But we can change our performance. That’s our focus now.”