Villarreal, 26, had struggled through most of the season, compiling a 5.42 ERA over 35 relief appearances. If he clears waivers, he will have the right to accept or refuse the Minor League assignment because he has five years of big league experience.
"We just need someone to be able to give us multiple innings," said Astros manager Cecil Cooper. "That's what we'd hoped Villarreal would do for us, but he just didn't have the consistency that we were looking for. He wasn't the guy that we initially thought he was when he first came over here."
If Villarreal refuses the Minor League assignment, the club will be reponsible for paying the remainder of his salary through 2009. The club's hope is that Villarreal will accept the assignment and go to Round Rock, where he will receive regular work -- something he was not afforded with the Astros.
"He basically has three choices," said Astros General Manager Ed Wade. "One would be to accept the assignment to play in Triple-A, an optional assignment. Secondly he could reject the assignment, in which case we would either have to give him his unconditional release or trade him. Third, he could take his free agency, which he's not going to do because in that case he's not going to get paid. He has three days to make his decision."
The club's hope is that Villarreal will accept the assignment and go to the Express, where he will receive regular work -- something he was not afforded with the Astros.
"We need to get him right," Cooper said. "The idea was to send him there and to let him work regularly. He has [three days] to accept that assignment, but that's the plan. There you don't have to worry about the wins and losses as much as you do here."
Wade spoke to Villarreal's agent, Oscar Suarez, who said he and Villarreal will discuss Villarreal's best option.
"When we talked to him we urged him and encouraged him to stay in the organization and go to Triple-A, spend some time with us, get straightened out and get back up here and help us," Wade said. "We think he can help us not only this year but for the future."
Borkowski, 31, was 1-1 with a 2.29 ERA over 12 appearances for the Express, including one start. He allowed five earned runs over 19 2/3 innings, walking five and striking out 12.
Borkowski started the season with the Astros, recording an 0-2 record with a 6.86 ERA over 18 relief appearances. He was designated for assignment on May 27.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.