Towles, 26, hit .191 (9-for-47) with one homer and eight RBIs in 17 games with the Astros. He was the Opening Day starter and wound up starting 13 of the team's first 26 games, splitting time with career backup Humberto Quintero.
The Astros have No. 1 prospect Jason Castro catching at Round Rock, so the club sent Towles to Double-A to give him a chance to play every day.
"I take it alright, I guess," Towles said. "I'm not happy about the situation. I wish I would be up here, but I guess they've got to make moves, and I'm that move."
When asked about his level of frustration, Towles said: "I'm more ticked off right now about catching a flight in the morning to go to Midland [Texas]."
Towles, who fizzled when he made the team out of Spring Training in 2008 as the club's No. 1 prospect, won the starting job in the spring over Castro, who was drafted No. 8 overall in 2008. He never got going at the plate, but he's certainly not alone.
"I felt like I did alright considering I only had like 40 at-bats," he said. "I'm not knocking the cover off the ball, but to be honest, nobody really is."
Towles said he's not sure where he stands in the organization.
"I have to talk to my agent and see the options I have and go from there," he said.
Cash, 32, was hitting .174 with two RBIs in seven games at Round Rock as Castro's backup. He is a veteran of seven Major League seasons with Toronto, Tampa Bay, Boston and the Yankees and is a career .186 hitter.
Astros manager Brad Mills said Cash's experience working with pitchers made a difference.
"I think everyone was able to see that in Spring Training and know what kind of catcher he is and know how he goes about that," Mills said. "Jason, we still want to get him more time to develop so when he gets here he's ready."