Jeff, the good news with Mercy is that it built week on week and is showing uptick over the season. Unfortunately it's still lagging against tough competition that night.
The potentially good news for the series is that the Leno experiment is failing so if they kill that and go back to their old model of various shows airing @ 10pm that's another 5 hours of programming that they need to fill meaning that they may be a little more forgiving to weaker rated series currently on the air if it shows enough potential.