Part of the reason Jon Singleton hasn’t been promoted to the majors yet might be because his potential free agency has not been pushed back yet.

The Astros top prospect at first base has a lot to prove after a disappointing 2013 that included a suspension for marijuana. He also had a poor spring training, so he and George Springer aren’t in exactly the same boat.

But, like Springer was, Singleton is doing very well at Class AAA Oklahoma City. The Astros’ offense has been miserable, and first base is a platoon situation in the bigs.

There’s another reason Singleton might not be called up yet.

Springer can’t become a free agent now until after the 2020 season. Singleton, were he promoted Wednesday, could potentially still become a free agent after the 2019 season.

Why is that?

Springer did not begin the year on the 40-man roster. But Singleton did, so when the Astros sent him to the minors, they technically optioned him.

MLB rules stipulate that a player who is on optional assignment for fewer than a total of 20 days in one major league championship season will be credited with service time for those days.

So if Singleton remains in the minors for 19 days and comes up and stays around the whole year, he’d receive a full year of service time. But, if he stays down for 20 days or more (they don’t have to be consecutive), he cannot receive a whole year — the most he could receive is 163 days of service.

MLB’s championship season started March 30, for all teams. It doesn’t matter when teams played their first game.

Spending 20 days on optional assignment then brings us to April 19 as the first day Singleton could be promoted and also guaranteed to be under control through 2020.

This holds true for any other player on the Astros’ 40-man roster who hasn’t been to the majors yet: i.e. Domingo Santana, an outfielder at Oklahoma City.

Said Astros general manager Jeff Luhnow on Wednesday: “(Singleton) does not have the Triple-A track record that Springer has yet. We feel he needs to develop that track record. We’re always going to be in a position where our fans want our top prospects to come up here sooner than we think they’re ready to come up here.”