Castillo, whom the Mets intend to pursue, and Paul Lo Duca, who could return to catch, were the two potential regulars among the six. Also filing were Marlon Anderson, Ramon Castro, Damion Easley and Shawn Green.
The Mets need Castillo to play second base, and though they hardly intend on re-signing Lo Duca, they see few alternatives among the potential free-agent catchers.
New York has more potential free agents than any other team, 14, but only two of the other eight -- Tom Glavine and Moises Alou -- may come to figure in the club's plans. The others included Sandy Alomar Jr. and Jeff Conine, who have said they plan to retire, Aaron Sele and Jose Valentin who, people familiar with the club's thinking, have no place in the Mets' plans, and Brian Lawrence, who was removed from the 40-man roster in September.
The club intends to exercise its option on Alou's contract. The deadline for doing so is Nov. 15.
Filing on the first day in no way affects the player's free agency. All free agents have until Nov. 11 to file. Once a player files, he can speak with all clubs, but negotiate and sign only with his incumbent club. Once the filing period ends, all free agents are eligible to sign with any clubs.
Castillo, who underwent surgery on his right knee two days after the season, and Glavine appear critical to the Mets. Although the Mets have alternatives at second base -- Easley, Valentin and Ruben Gotay -- none is the defender or the No. 2 hitter Castillo is.
The Mets have interest in Easley and Gotay as reserve players. Valentin would return only if the club was convinced he could be an adequate extra outfielder if a need developed. Even then, there is little chance of him returning.
The Mets have no sense of what Glavine intends to do, and neither does he at this point. He declined to exercise his contractual option for 2008 five days after the season ended, but didn't discount the possibility of re-signing with the Mets. The club, however, suspects that Glavine would prefer to re-sign with the Braves if he, in fact, decides to continue his career.
Anderson is a favorite of manager Willie Randolph, as well as a remarkably productive left-handed pinch-hitter. His presence likely would eliminate the chance of Green returning. But there were indications Monday the Mets won't immediately turn away from Green.
Castro was quite productive in his role as Lo Duca's understudy last season. And Easley was in the midst of a relatively productive season when he suffered a severe sprain of his left ankle on Aug. 18.
Marty Noble is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.