The Portuguese forward recently completed a €105 million move to Serie A champions Juventus, ending his eight-year stay in Spain.
Ronaldo topped Los Blancos' scoring charts last season with 44 goals in all competitions and Real are yet to bring in a big-name replacement.
"His 50 goals a season are not easy to replace," Kroos told Kicker.
"Cristiano has been an important building block in our team over the past few years and a decisive factor in our success.
"But we will be hard to beat again this year."