There was confusion following the introduction of the Newcastle debutant in the 54th minute as he charged around midfield before being instructed, by Bruce, to change position within 60 seconds.
And Willems was then caught on his heels at the onset of the move which led to Pierre-Emerick Aubameyang's 58th-minute strike.
"I put Jetro on in midfield, where he played all last season for Frankfurt, and then I very quickly thought, 'Do you know something? That's a bit unfair on him'," said Bruce.
"Maybe I should have done it beforehand. It's a bit unfair on him because he's making his debut and (I thought), "I will put him on in his favoured position", so I switched him very, very quickly.
"It's unfortunate, but that's the way it was."