United languish in ninth place in the Premier League following 1-0 losses to Everton and Newcastle United in the space of four days and have made their worst start to a domestic season for 12 years.
Despite having already qualified for the last 16 of the Champions League, Moyes will find himself under intense scrutiny when United host Ukraine's Shakhtar Donetsk on Tuesday needing a point to guarantee top spot in Group A.
"The players are hurting because they are used to winning," the United manager told a news conference on Monday.
"When we don't win that hurts. The expectancy is to win all the games. They'll respond in the right way.
"I take complete responsibilty for the results.
"Fortunately they have been good in the Champions League but not so good in the Premier League and I will take responsibilty for them as well.
"We've lacked a bit of good fortune which could have made all the difference."
United's 5-0 defeat of Bayer Leverkusen two weeks ago sealed their progress from Group A but Shakhtar can join them with a victory at Old Trafford.
Moyes has injury concerns over defenders Nemanja Vidic, Patrice Evra and Chris Smalling although Japanese midfielder Shinji Kagawa could be available after needing to have his stomach pumped because of illness after the defeat by Everton.
"Shinji trained today and I think he'll be okay tomorrow," a tense-looking Moyes said.
"We have got one or two injuries that I need to look at and that will come into consideration."
