The remaining contenders to join a rotation already occupied by Roy Oswalt, Brandon Backe, Wandy Rodriguez and Shawn Chacon are Woody Williams and Chris Sampson.
Cassel, 27, was 1-2 with a 4.85 ERA over five games this spring, including two starts. He allowed seven earned runs over 13 innings.
Although he was disappointed that he won't make the team out of Spring Training, Cassel was encouraged by the vote of confidence he received from general manager Ed Wade and manager Cecil Cooper when they broke the news that he was cut.
"Basically, they reiterated that they felt I did a great job," Cassel said. "I opened some eyes in my time here. In the innings that I got, I took full advantage of the opportunity I was given. They're looking for me to go down, have success at Triple-A, continue to pitch and get innings, and fingers crossed, I'm the name they call when they need somebody."
The Astros made two roster moves on Wednesday. In addition to cutting Cassel, they reassigned right-hander Runelvys Hernandez to Minor League camp.
Hernandez will accompany the team to Houston on Thursday and will be available to pitch in the exhibition games at Minute Maid Park on Friday and Saturday.
Hernandez was removed from starting pitching consideration more than a week earlier, but until Wednesday he was still in contention for a bullpen spot. The 29-year-old right-hander appeared in six spring games and posted an 8.31 ERA, allowing 12 earned runs over 13 innings.
Hernandez said he has an out clause in his contract, which states he can become a free agent if he's not on the club's roster by March 31, and again on May 15. The right-hander indicated he does not plan to exercise the clause, and after taking a day off on Wednesday will rejoin the club.
"I was surprised," Hernandez said. "Lately, they've been talking about my arm, everybody sees I have a pretty good arm. I'm showing them I'm getting better every outing I'm pitching. I feel happy, but at the same time, I feel sad because I was thinking I'd make the club.
"I belong in the Major Leagues, not the Minor Leagues. It's fine. I'll keep working hard and I know I'm going to see them soon."
The competition between Williams and Sampson for the fifth starter spot will likely go down to the wire, and the Astros may not decide between the two until Saturday, after the final exhibition game of the spring.
The veteran Williams has struggled this spring, but he had a good showing in his most recent outing against the Braves on Sunday, allowing three runs over six innings. Williams is slated to appear on Friday, following Rodriguez's start.
Sampson exited his start on Tuesday in Lakeland, Fla., early after a Miguel Cabrera line drive hit the right-hander on his right hip, but Sampson said on Wednesday that he felt better and was planning to throw his scheduled bullpen session the next day.
The Astros have 30 players remaining in big league camp who are eligible for the 25-man Opening Day roster.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.