In my head canon I have Decker serving aboard the Enterprise in the last year of its first 5YM. That way at least Decker has some space experience and Kirk can know him and groom him as a protege. Then when Kirk is promoted he can say 'I recommend you put Will Decker in command, he's a fine young officer and worthy of the opportunity'
In early drafts of the script, it was established that Will Decker had made his name as first officer of the USS Boston.