Amy Gutierrez of CSN Bay Area gave more specifics on the injury.
Cueto left the game in the first inning with Marco Scutaro at the plate and having recorded only one out.
It isn't known for sure, but Cueto did have to start and stop his delivery just a few pitches before leaving the game as a result of Angel Pagan receiving a timeout by the home plate umpire.
This is a huge blow for the Reds. As the team's ace, Cueto sported a 19-9 record with a 2.78 ERA this season. If he misses an extended amount of time, the Reds will have to beat the Giants and potential future opponents without their ace.
More importantly, Cincy can no longer match San Fran's ace, Matt Cain. But back spasms are never a serious injury and Cueto has a very realistic shot of returning in this series. However, the torque of Cueto's delivery could quite possibly keep him out longer than most pitchers who suffer the same injury.
That, of course, will all depend upon how serious the injury actually is.
Manager Dusty Baker could have relied on his bullpen to finish this game, but he was wise not to do so. Mat Latos came into the game in the third inning, replacing Sam LeCure, who has been a reliable reliever for the Reds this season.
LeCure did a nice job, throwing 1.2 innings of scoreless ball for Cincinnati, handing Latos a tie.
Latos was scheduled to pitch in Game 3 of the NLDS on Tuesday, but you can scratch that now. Latos will be pitching on short rest as he last threw on October 2, going five innings against the St. Louis Cardinals.
Stick with Bleacher Report for all the latest on Cueto's injury.