"We are really disappointed," he told the club's official website. "We spoke before about what the game meant, how we had to start and what we had to do, but we didn't play football today.
"Our strength is playing football. I watched the Norwich game in midweek and you could see how good we were, but today we just didn't get going. Sunderland played well at times though and got the goals.
"I'm happy to be here and to play, but it is not the beginning I hoped for. I wanted to win this game. It's a terrible debut for me."
De Jong arrived on loan from Borussia Monchengladbach on Wednesday.