The 22-year-old has apologised for his mistake that allowed the Portuguese side to score their second goal in a 3-1 Europa League victory over the Magpies in Lisbon
The Italian defender was at fault as he played a sloppy back-pass to goalkeeper Tim Krul that was intercepted, resulting in Benfica’s Lima scoring to put the Portuguese side ahead.
The Magpies now need to score at least two goals in the return fixture when Benfica travel to St James’ Park if they are to have any chance of progression.
"I am feeling angry with myself," Santon, the left back, told The Telegraph.
"I think I had a good game against Benfica, but that pass was a disaster. As a defender, you have to make sure your pass is right, but I had Mapou Yanga-Mbiwa near me, so I couldn’t see the striker. I am really angry with myself and when I got in the dressing room, I said sorry to everybody, to all the players, because it was my fault.
"Now we have to forget about it. I have to forget my mistake and think about Sunday and then, next week, we have to score two against Benfica."
Newcastle have league matches against Fulham and local rivals Sunderland before the return fixture, but Santon knows that focus is required.
"The Europa League is important, but the Premier League is even more important," he said.
"We need to make sure we save our season and stay in the division."