Rafael Benitez's complaints regarding a lack of signings will only be heightened following a match that saw Newcastle dominate but fail to convert their chances.
Newcastle lost Mohamed Diame to a first-half injury, but equally concerning for Benitez would have been his side's inability to put the ball in the net.
Within 40 seconds of the second-half restart, Atsu's low cross found Ritchie, who in turn teed up Perez to flash one past the upright.
The game then went a bit sloppy, with home debutant Fabian Schar, Jonjo Shelvey and Jacob Murphy all being brought on for the Toon, and moments later Augsburg - 12th-place finishers in the Bundesliga last season - were celebrating a breakthrough goal.
Towering forward Gregoritsch met Phillip Max's deep delivery to head what proved to be the winner past Darlow.
Darlow produced a good stop to deny Gregoritsch a second less than a minute later, but Newcastle also came close to a goal as Murphy's volleyed cross deflected onto the bar.
Murphy was then kept out by Luthe down low from the last of United's chances, meaning a bad end to a difficult pre-season.
NEWCASTLE UNITED: Darlow, Yedlin, Lascelles, Clark, Dummett, Diame, Hayden, Ritchie, Perez, Atsu; Joselu
AUGSBURG: Luthe, Schmid, Gouweleeuw, Callsen-Bracker, Max, Hahn, Khedira, Gregoritsch, Baier, Richter, Ji Dong-wong