It's less than seven weeks until the Astros active roster can be expanded to 40 players on September 1. At the moment, the club's 40-man roster is at full capacity. Several players who might be considered for a September call-up aren't currently part of the 40-man roster. This means someone would have to be moved off to make room for them.
So, if J.D. Martinez, Jose Altuve, Dallas Keuchel, J.B. Shuck or Andy Van Hekken (you knew I had to mention him, didn't you?) are going to make it to Houston before the season ends, someone will need to be cleared to make space.
A few candidates:
Brandon Lyon. Lyon is the no-brainer. He is currently on the 15-day disabled list. He could be moved to the 60-man DL and thus be removed from the 40-man.
Blake King, RHP, Corpus Christi. King was picked up during this season and added to the 40-man roster. He would seem to expendable.
Lucas Harrell, RHP, Oklahoma City. Houston just picked Harrell up a few weeks ago, so they would probably like to see a little bit of him in action before they let him go.
Another option for adding 40-man space is trading big league players for prospects who don't go on the 40-man roster.