In an average year there are 183 service days in a major-league season, 162 games and 21 off days. If you accrue 172 days in any season, it is considered a full year. Six full years gets you to free agency. Five years and 170 days does not.
I think this is the main reason why we traded for Tucker: so that we have OF options on our major league for those first 12 days (10 games) while we keep Ronald Acuna in AAA and preserve another full season of team control over him.
Under no circumstance should Acuna be on the major league roster come opening day... none. It would be the most idiotic business decision to make. The Cubs kept Kris Bryant in AAA for those first 12 days back in 2015 and have an extra year of team control over him as a result, despite the fact that he was the best player in Spring Training that year.