United sealed their first away win of the season by coming from behind to win at Ipswich for the first time in 12 years and climbed into the top six.
"You have to believe in yourselves and we did that," McDermott told reporters. "We have a real good spirit as well and you can't come from behind like that if you don't."
The 2-1 win at Portman Road maintained Leeds' unbeaten start and they head into next weekend's home game against Harry Redknapp's QPR with plenty of confidence.
"It's been a good start," added McDermott. "We got a draw at Leicester and win at Ipswich.
"We've taken eight points from four games which is a nice average.
"But there's another 42 games still to go and we're not looking beyond the next one, so there is no question of anyone getting carried away or thinking too far in front."