The Hammers went down to a first-minute Robin van Persie strike but largely restricted the hosts' chances and came close to nicking a goal themselves.
And the 31-year-old believes the east London side can go one better against Chelsea at the weekend if they maintain the same level of performance.
"If we play like that against Chelsea and are more clever in front of goal, we have a chance to win the game,” Demel told the club’s official website.
"Chelsea are a similar team to Manchester United, but they were worried last night. You couldn't hear their fans. At home this weekend I think we will push forward more.
|THE RAFALUTION GETS HAMMERED|
|16/5||West Ham are 16/5 with Paddy Power to beat Chelsea next weekend|
"We don't need to hide because we have an opportunity to win the game."
Manager Sam Allardyce was forced to call on several academy players for the trip to Old Trafford due to a spate of injuries but has received a boost ahead of the clash with the Blues.
“We have been stretched for a while," Allardyce told reporters. "My players ran their hearts out to stay in the game and we did put fresh legs on but after those we used, we are into the kids.
"At least Mark Noble is back for Saturday, which is important because we are tired."