Medical examinations showed the right back has lymphoma, a type of blood cancer, and he will now have several months of treatment, Gladbach added.
"The cancer has been detected extremely early and is treatable with medication," the club said in a statement.
"There is an extremely high chance that he will get back to full health and a normal life including professional sport could still be possible."
Lainer joined Gladbach in 2019 from Austrian top-flight side Salzburg. He has made more than 120 appearances for the German club in all competitions.
"We will do all we can to get Stevie the best possible treatment. We wish him and his family lots of strength and optimism in the fight against this disease," said Roland Virkus, board member for sport at Borussia.
Gladbach, who finished 10th last season, kick off their 2023-24 campaign at Augsburg on August 19th.
