Castro and Valbuena are each in their final year of arbitration before becoming free agents, and Gonzalez, Keuchel, Fields and Gattis are going through the process for the first time.
If the players can't negotiate deals with the Astros by Friday, they will exchange desired salary figures for 2016. The midpoint between what the player wants to be paid and what the team wants to pay him generally serves as a launch point for contract negotiations.
If the two sides can't agree to a deal by February, their cases would be heard by a three-person arbitration panel, which would side with either the player or the team. Arbitration hearings could lead to bad feelings, so both sides typically work hard to avoid taking the process that far.
The Astros haven't had a case go to a hearing since outfielder Hunter Pence won a ruling for $6.9 million prior to the 2011 season. The Astros had offered to pay him $5.15 million. Prior to Pence, the Astros hadn't lost an arbitration case since 1996, winning four in a row.
Keuchel last year was the third player in franchise history to win the Cy Young Award, joining Mike Scott (1986) and Roger Clemens (2004), who both won it in the National League. Keuchel went 20-8 with three complete games and a 2.48 ERA in 33 starts last season, and he led all AL pitchers in wins, WAR (7.2), innings pitched (232) and WHIP (1.017), and he ranked second in ERA and opponents' batting average (.217).
Keuchel, who made barely above the Major League minimum at $524,500 last year, could explore a long-term deal with the Astros at some point, but neither side seems eager at present. He's not scheduled to become a free agent until 2019.
Castro, meanwhile, hit .211 last year with 11 homers and 31 RBIs. He started 102 games and excelled defensively, throwing out 24 of the 66 runners (32.3 percent) attempting to steal. He made $4 million last year.
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.