Stuttgart qualified for the Europa League after securing a top-six finish in the Bundesliga last term, but have had a difficult start to their current campaign.
Bruno Labbadia's side have lost twice, including a 6-1 hammering against Bayern Munich, and drawn the other of their first three games and are languishing in 16th place in the table.
To add to Labbadia's woes he is also without Zdravko Kuzmanovic through suspension, while injuries will rule out Tim Hoogland as well as intermittently Cristian Molinaro and Johan Audel.
Captain Serdar Tasci is also set to miss out again due to a thigh injury.
Labbadia has praised the Romanian league leaders as a "well organised side", and Hajnal knows it may take time for the hosts to break down their opponents, but believes if they retain their focus the result can go their way.
He said: "They are an opponent with plenty of experience in European competition and they will probably take a more defensive approach.
"We will need to be patient and concentrated in putting our game together to make sure we come away with a good result."
Stuttgart have never lost to Romanian opposition, while Steaua have not beaten a Bundesliga side in 55 years since a victory over Borussia Dortmund during the 1956-57 season.