Dieudonne Mbokani put Liege ahead before Yakubu fired in an equaliser but Everton's Joseph Yobo put a cross into his own net before Castillo's 25-yard shot again hauled the Blues level.
Moyes said: "We will have to play well to win in Liege in a fortnight in the second leg.
"This was always going to be a difficult tie, but we have made it more so now."
Moyes, who confirmed that the final details of his new five-year contract were now being tied up by the club, still insists that all is not lost in Europe.
He continued: "We have been in this position before. Last season we reserved our better performances for the away legs in Europe.
"But we have been punished for bad defending. The game was open, too open, for us.
"The defending was poor and that is what cost us tonight. Joleon Lescott is having a run when he had one good game and one poor game.
"Now we know we have to go to Liege and score. And we will have to play well. But all is not lost, we have won ties from this position before."