The Royals have struggled since returning to the top flight as Championship winners and have collected just one league win so far this season.
That victory came against Everton in mid-November and Reading have since suffered four successive defeats, leaving them 19th in the standings.
Brian McDermott's side are four points behind Wednesday's opponents Sunderland, who occupy the place directly above them, and defeat would see that gap widen - something that worries left-back Shorey.
"It concerns me a lot, yeah," he said. "There is not a lot else you can say.
"It concerns me we're down there but we've got to man up and make sure our performances get to the level they need to be because they haven't been.
"Every game is big, especially where we are. It's not about who you play and where they are in the league, it's all about the performance.
"If you get it right, then the results will come."
Shorey concedes Reading did not get their performance right on Saturday against another side at the wrong end of the table.
Jason Puncheon's second-half strike proved enough for Southampton to secure three points from a game that the Royals never looked much like levelling things up in.
"It's a good thing we've got a game coming up quickly so hopefully we can get over it," Shorey said.
"We see three points as a minimum requirement every game.
"We feel like we can get something out of every game, but not with second-half performances like that [against Southampton]. We'll be up for Tuesday."
Reading will be missing a number of important players at the Stadium of Light, including Jem Karacan, Jimmy Kebe and Alex McCarthy.
Chris Gunter is almost certainly another out, joining Pavel Pogrebnyak and, most likely, Sean Morrison on the sidelines.
The latter looks set to miss out after leaving Southampton on crutches due to a knee injury picked up in the second half at St Mary's.