The Astros agreed to terms on a three-year, $60 million contract with Walker on Friday, Bob Nightengale of USA Today reports.
Walker rejected the Diamondbacks' one-year, $21.05 million qualifying offer Tuesday, Mark Feinsand of MLB.com reports.
The Diamondbacks extended Walker a one-year, $21.05 million qualifying offer Monday, Jeff Passan of ESPN.com reports.
Walker went 2-for-5 with a double, an RBI and two runs scored in Wednesday's 8-2 win over the Giants.
Walker went 1-for-4 with a double and an RBI in Monday's 6-3 loss to the Giants.
Walker is not in the lineup for Sunday's game against the Brewers.
Walker is out of the lineup for Sunday's game against the Astros.
Walker started at first base and went 1-for-4 in Thursday's 3-2 loss to the Giants.
Walker is out of the lineup for Wednesday's game against the Giants.
Walker went 1-for-5 with an RBI in Tuesday's 8-7 win over the Giants.