Valbuena was 33, and Castillo was 37.
Valbuena and Castillo were in their native Venezuela playing winter ball with Cardenales de Lara. They both appeared in Thursday's game and were traveling together at the time of the accident.
Valbuena, Castillo, and another Cardenales de Lara player, former major-league infielder Carlos Rivero, were in the car that was being driven by Rivero's driver. The accident happened when the car hit a rock in the middle of the road, and Valubena and Castillo were ejected from the vehicle, according to The Athletic's Fabian Ardaya, citing Venezuelan news reports. Both Rivero and the driver survived the accident.
Valbuena made his major-league debut in 2008 with the Seattle Mariners and played 11 seasons, spending time with the Cleveland Indians, Chicago Cubs, Houston Astros, and Angels. He appeared in 96 games with Los Angeles last season before his release in August. He was a free agent this winter.
"I will miss Luis' banter, smile, genuine love for his teammates, and, of course, the bat flips," Astros manager A.J. Hinch said in a statement to Brian McTaggart of MLB.com. "He was a beloved person whether he was on our team or across the field. My thoughts and prayers are with his family and the lucky ones who could call him a teammate or friend."
Castillo made his MLB debut in 2004 and played parts of four seasons with the Pirates. He split his final year in the majors between the San Francisco Giants and Astros in 2008; since then, he'd spent time playing in Japan, Mexico, and Venezuela.