Reina has been the Premier League's top keeper in recent years, winning the Golden Gloves clean sheet award in four of the past five seasons. But the Kop stopper was caught out at his near post for Hammers' third goal on Sunday as Liverpool lost 3-1.
And Reina knows he must raise his game at Anfield this Sunday if the Reds are to put a dent in United's hopes of a record 19th league crown.
Reina, 28, said: "I won't hide behind any team-mates - I was poor against West Ham. "I have to recognise this and keep learning from my mistakes.
"Keeping those clean sheets was about not making any, first of all. "I made a couple of errors on Sunday and felt really disappointed with my display."
Those three West Ham goals ended Reina's run of six lockouts in seven games but the Spaniard still lies just two clean sheets off pacesetter Petr Cech, who has 12 for Chelsea.
And Liverpool will certainly fancy their chances of giving their fiercest rivals another kick in the guts following Chelsea's comeback win over United on Tuesday.
United, already missing the crocked Rio Ferdinand and Jonny Evans, will also be without Nemanja Vidic after his Stamford Bridge dismissal.
That means rookie Chris Smalling and Wes Brown or John O'Shea will form the backbone of the defence, increasing £35million signing Andy Carroll's debut chances.
Carroll is finally in full training following a thigh injury, and Kenny Dalglish will decide late in the week whether to unleash the giant striker on a makeshift backline.
Reina certainly hopes so and insisted: "I hope it turns out like we all think it will. He's a great player, a top striker and he will score a lot of goals for Liverpool.
"He has that physical condition. Andy is a massive unit and a tough bloke to deal with. I know from my own experience how difficult it is to face these kinds of opponents."