VerbRelevant meaningFunction
رَجَعَHe returnedBasic lexical sense
اِسْتَرْجَعَHe pronounced the known phrase of return to AllahCondensed expression
The root ر ج ع concerns return.
Translation: The root meaning returning.
However, in this textbook's example, istirjāʿ refers to speaking a complete established formula. The basic root meaning is not an adequate replacement for the lexicalized derived usage.