The Bernabeu boss has regularly changed between Karim Benzema and Gonzalez Higuain this season, but Valdano believes this policy is affecting the pair's form.
Benzema has scored six goals from 10 starts whilst Higuain has eight from 12.
Valdano said: "There comes a time when you are having to prove yourself every day, you begin to doubt yourself.
"Neither player has won the race. Some weeks we see a step forward for Higuain, other weeks Benzema looks to be ahead. If we look at the statistics, the two are getting fewer goals than last season.
"I encourage them to decide, not even the fans know who is starting each game, and that can be uncomfortable for the big stars."