Demel was substituted early on after colliding with team-mate Roger Johnson, his injury leading to 10 minutes of stoppage time at the end of the first half.
The defender's absence did not affect Sam Allardyce's men, who went on to claim a hugely valuable three points thanks to goals from Carlton Cole and Mark Noble.
West Ham's victory provided a positive end to a week that had seen the club ship 11 goals in cup defeats to Nottingham Forest and Manchester City.
Assistant manager McDonald is confident that Demel, who is set stay overnight at a local hospital, will recover quickly to hand West Ham another boost.
"Guy Demel has had a bad whack on his head off Roger Johnson's elbow; we're slightly concerned with him," McDonald said.
"He's been taken to hospital. We're just waiting for the doctor to come back to give us an idea of how he is but I'm sure he will be fine."
McDonald believes West Ham should take confidence from the win at Cardiff, a first triumph in seven across all competitions that moved the London club out of the Premier League's bottom three.
"All the staff and all the players have been hurting because we haven't performed to how we know we can," he added.
"But we've shown lots of character under severe criticism in the last couple of weeks and it's good that we've come out the other side.
"Hopefully that will give the players a lot more confidence."
