Santon revealed the visitors went into the game with new-found confidence after their win over Queens Park Rangers last weekend and were disappointed not to have taken advantage of it.
He told the club's official website: "We had a really big chance to win this game, but the last 10 minutes were good for Manchester United and they won, while we come back to Newcastle with zero points.
"I think people thought Newcastle would go to Manchester United and come back with zero points, but we really had a chance to win the game.
"We were confident because last week we won against QPR, so when you win, for the next game you always have confidence. You have a winning mentality.
"Today was like that. We came here to get the result, but in the final minutes we were beaten and we are really disappointed because we should have won."