Koch has not featured since the opening weekend 5-1 defeat at Manchester United, the German international troubled by a pelvic issue which required minor surgery in the USA.
Koch's return had then been delayed by a bout of illness but the German comes straight into the XI as one of four changes to replace Llorente who is not involved.
There are also starts for Gelhardt, Drameh and Mateusz Klich as injured duo Jamie Shackleton and Dan James plus the suspended Junior Firpo drop out of the side.
Leeds were already without injured quintet Kalvin Phillips, Patrick Bamford, Liam Cooper (all hamstring), Rodrigo (heel) and Pascal Struijk (foot) for Tuesday night's clash at Manchester City in which the Whites fell to a 7-0 defeat, the club's heaviest Premier League loss.
Shackleton was also forced off injured during the contest with an Achilles problem and James then suffered a groin injury in training.
Charlie Cresswell is now also missing, the 19-year-old defender having injured his shoulder according to a national media report.
Fifteen-year-old Archie Gray is among the Whites substitutes, the average age of which is approximately 18 years and eight months.
Arsenal, meanwhile, have named an unchanged side for a fourth game in a row, Emile Smith Rowe having to settle for a place on the bench.
Striker Pierre-Emerick Aubameyang is not involved having been stripped of the Gunners captaincy following disciplinary issues.
Goalkeeper Bernd Leno returns to the bench from a recent groin issue but defender Sead Kolasinac is out with an ankle injury, the left back expected back in January.
In the absence of Aubameyang, Alexandre Lacazette once again captains the Gunners whose XI features former Leeds loanee Ben White.
Leeds United: Meslier, Drameh, Ayling, Koch, Dallas, Forshaw, Klich, Raphinha, Harrison, Roberts, Gelhardt. Subs: Klaesson, Moore, Kenneh, Jenkins, Bate, McCarron, Gray, Summerville, Greenwood.
Arsenal: Ramsdale; Tomiyasu, White, Gabriel, Tierney; Partey, Xhaka; Saka, Odegaard, Martinelli; Lacazette. Subs: Leno, Holding, Chambers, Tavares, Cedric, Elneny, Pepe, Smith Rowe, Nketiah.
Referee: Andre Marriner.
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Thank you Laura Collins