The Whites boss lauded the opposition's finishing, but was adamant that Matt Smith should have won a spot kick when he tumbled in the area with the score at 1-0.
"Their first goal, from their point of view, was a little bit of quality on the left-hand side and maybe we needed to do better in that situation, but it was a good header, and their second goal was a great finish," BBC Sport quotes him as saying.
"Having said that, at 1-0 Matt Smith was bundled over in the penalty area and I think because he is a big lad, the ref didn't give it. But it was a certain penalty, there's no doubt about that."
Newcastle will entertain Manchester City at St James' Park in the next round.