Both defenders joined the north London side in January and are yet to make their debuts for Arsene Wenger's first team.
In training, though, the duo looked sharp, as 17-year-old Bielik (pictured above) went in for a crunching tackle (not too crunching during practice) on Mesut Ozil and Gabriel (below) tested out his pace during sprint sessions.
Arsenal will be looking to go all out in defence of their FA Cup trophy, with the possibility of a Premier League title - realistically - all but over.
At the same time, however, the Middlesbrough clash presents Wenger with the perfect chance to hand Bielik and Gabriel their debuts.
As important as the FA Cup likely is to Wenger, no points can be dropped and it may be worth testing out a different competition in a one-off scenario.
Championship opposition also present a different type of game to sides in the top flight, although Middlesbrough's form this term could easily have supporters arguing they could match those in the Premier League.
It should be an intriguing clash, of course, but will there be added intrigue for Arsenal fans with two debutants to keep an eye on - or will Wenger play it safe v Middlesbrough?