He was assigned to Triple-A Sugar Land on Saturday.
"He's a potential bat that can help us," Astros manager Dusty Baker said. "Hopefully, he can unmask that talent that he has and at some point in time help us."
The 31-year-old was regarded as one of the top infield prospects while with the Astros organization from 2011 to '17 after being acquired in a trade with Philadelphia.
Singleton played in 114 big league games with Houston during 2014-15.
He appeared in 11 games with the Brewers this season but has spent the majority of the season with Triple-A Nashville. After being designated for assignment on Wednesday, he chose to become a free agent.
Singleton has hit .252 with 169 home runs, 645 RBIs and an .828 OPS in 1,064 minor league games.