The Lions offense was not clicking in that game. Since they've started firing on all cylinders (whenever you define that as being but certainly after week two), they haven't lost a game.
The Rams offensive line is still a mess. They've benched their blue chip free agent Jonah Jackson after they realized they didn't have a role for him, and have lost games to the Bears (in week four) and Dolphins (in week ten) this season, and have been smoked by the Eagles at home just a few weeks ago.
If they're playing at their best, they're a playoff team, and no playoff team is an easy out. But I would expect a third rematch to be along the lines of what the Lions/Bucs game looked like in the Divisional Round last year, where it's set at a -6 to -7 spread, and if the Lions play their game, they win it far more often than not.