Arsenal manager Unai Emery says "the key is in Mesut Ozil's hands" as the midfielder tries to regain his first-team place.
Ozil, 30, is the Gunners' highest-paid player, having signed a £350,000-a-week contract in 2018, but has started only 13 of 26 league games this season.
"He needs to be consistent in training and for matches, without injury or being sick," Emery said.
"Like that, we can see the best Mesut with us."
Ozil has played 216 times for Arsenal since joining from Real Madrid for a then club-record £42m in 2013.
But he has started only one game since Boxing Day, with injuries and "tactical reasons" given as explanations for the German's absence during the campaign.
The Gunners trail Bate Borisov 1-0 from the first leg of their Europa League last-32 tie and speaking before Thursday's second leg, Emery said: "The key is in his hands and he is working very well this week."