During the offseason of 2009, the Cubs and Mets discussed a trade that would send outfielder Milton Bradley to the Mets in exchange for Castillo and another player.
If the Cubs still were interested in the 35-year-old Castillo, they would have to know that his attitude as well as his playing ability were still there.
Mets manager Terry Collins talked to Castillo at one point about what he deemed "sullen behavior," according to an MLB.com story.
The Mets will absorb the $6 million remaining on Castillo's contract. That means any team can sign Castillo to a new contract at any agreed upon price.
Castillo hit .235 in 86 games last season. He is a three-time All-Star, three time Gold Glove winner with a .290 lifetime average and 370 stolen bases.
The Cubs' second base situation is still somewhat unsettled with two weeks to go before Opening Day. Of the four candidates, only Jeff Baker and Darwin Barney have performed well offensively and defensively.
The only left-handed hitter in the mix is Blake DeWitt, who has struggled in the field and at the plate.
"I think there are very few guys who work harder than [DeWitt]," manager Mike Quade said. "I know he's disappointed he hasn't played better. Whether he's relaxed or not getting pitches, or what's going on inside during a game ...
"He's been around a while and he's really focused on second base. There are all sorts of things he's had to deal with. We look for him to keep swinging the bat and work his way out of this, as well as get better defensively. Then we'll see where we're at in a few weeks."
Veteran infielder Auggie Ojeda has missed time with back spasms. He can be asked to go to the minors as part of the contract he signed. Quade said there's no timetable for Ojeda to get back on the field at this point.
Barney also has minor-league options.