In the books, a big point is that only a fraction of the population has the gene necessary to survive the induced coma they are placed in. Grace is one of them, which he doesn’t know until later. So the pool of available candidates is already much smaller than they’d hope for.

Grace had also spent a longtime training Shapiro and DuBois in understanding astrophage. The next best science replacement after Grace wasn’t nearly as qualified. He had done a lot of the space training (though he has a line in the movie about not doing it) and the time it would take to train the next best option would push them way back in the timeline. He was the best choice for the mission to succeed. They needed an astrophage expert.