Rangnick manages the towering forward for the Austria national team.
Kalajdzic, 25, has been linked with a move to Old Trafford and would be available for £20m.
However, staying at Stuttgart would be better for his development, according to Rangnick.
"It is important that Sasa stays injury-free this season and trains and plays at the highest possible level," Rangnick told Sky Austria.
"If he stays in Stuttgart this season, it will not be a disadvantage for his development."