The former Arsenal attacker had to be withdrawn just before half time against RB Leipzig at the weekend due to adductor fatigue, and while Julian Nagelsmann is hopeful that he will be fit for midweek, the Bayern boss may decide against risking him from the first whistle.
Should Gnabry not be given the green light to feature, there will be an opening for teenage sensation Jamal Musiala, who found the back of the net during yet another impressive cameo on Saturday to make it four goal contributions in four Bundesliga games.
Leroy Sane and Robert Lewandowski also netted against Leipzig and are set to retain their places here, especially with Kingsley Coman struggling to overcome a calf problem.
Marcel Sabitzer's first appearance in a Bayern shirt came against his former club on Saturday, but the Austrian lynchpin has his work cut out to force his way into the team here, with Thomas Muller boasting six goals from five games against Barca.
Corentin Tolisso is another absentee with a calf issue, but the out-of-favour Frenchman would not be in with a chance of displacing Joshua Kimmich or Leon Goretzka either way.
Benjamin Pavard and Lucas Hernandez were unable to link up with France for the international break, but both men have returned from injuries to bolster the Bayern defence at a crucial point.
Bayern Munich possible starting lineup: Neuer; Pavard, Upamecano, Hernandez, Davies; Kimmich, Goretzka; Sane, Muller, Musiala; Lewandowski