The right-back failed to deal with a cross as George Boyd was afforded space to head home the winner on Saturday, and the manager has called for more consistent performances
The Spaniard failed to jump for a cross six minutes before the break, allowing George Boyd to head past visiting goalkeeper Michel Vorm.
"Angel has to do better than that at the back post," said Swansea boss Monk, who played alongside Rangel for several years.
"It was too easy. You have to be stronger than that; you have to be willing to get hurt. That's what you have to do at the back post.
"You knew that cross was coming. It's not like he didn't expect it. And we've been working all week on those situations - that's the disappointing part for myself."
Monk feels his side need to do more to eke out points when they are below their best.
"If you can't win games, make sure you don't lose them," he said. "When you have bad periods in games, you do all you can to make sure you don't lose that game. We didn't do that today."
"The best teams in the top leagues consistently perform well. Unfortunately, throughout this season, we've been far too inconsistent. It's very disappointing on the back of two very good performances.
"We were very poor first half. It wasn't what I expected. In the second half, we were marginally the better team. We probably did enough to get a draw, but we can't grumble about them getting the three points.
"It was a poor game overall. We didn't string passes together or do enough to win the game. Neither team did much to win the game."