It's not about whether the injury could happen to a seasoned pro - obviously any time a player steps on the pitch he could get injured. The point was that young players, whose bodies are not far along in their development, are much more likely to derail their careers and put a lower cap on their ceiling if they pick up injuries (usually they're niggling as opposed to severe) because their bodies aren't yet built to withstand the grind of a first team football season. Speaking from experience, if you pick up an ankle injury as a young player, that ankle becomes a problem area for the rest of your career. And the more you pick up niggling injuries in that area, the more you compensate with other muscles, which leaves you at risk of other, more serious injury in other muscle groups.
As I said, if the staff believe Garner is physically ready then he ought to be given a chance. But not every player develops physically at the same rate - it took me until I was 20 to really start putting on bulk, despite being on a lifting schedule for my university's team from the time I was 18. Saying "sink or swim" is so black and white and does not leave any room for variability based on the facts and circumstances of each case.