"He may not be the player he was five or six years ago, but that doesn't mean he can't play," said Mattingly.
"Why get on Fukey? I know this is about the bottom line, but I get upset with Fukey-beating. I believe in the guy and what he's capable of doing. I see bat speed. He's just trying too hard. You love that, it's better than not trying. You can't defend the no production, it's right in front of you, but I can defend effort."
Jamey Carroll, in a slump himself, with a .190 average for July, started at shortstop on Sunday.
Furcal, who spent time on the disabled list with a strained oblique, is 3-for-34 since returning on July 3.
"You guys get on Fukey," Mattingly said to reporters, "but he's still got bat speed. Last year [the Mets] were ready to get rid of Beltran. But [hitting coach Jeff Pentland] and I saw he still had bat speed, and he's back rolling and he's big trade talk. Fukey is in the same boat. He's still got bat speed. He's been hurt so much. I can't ever get away from this guy playing with energy every time. He's never not trying."
Mattingly said that the same applies to another struggling infielder, Juan Uribe, who also started on Sunday.
"Juan's had a tough year, but last year was a career year, and it's not going away in three or four months," Mattingly said. "He's got to find it. We can work with him, talk to him, but until he squares up a ball in a game, it's hard to get it back.
"There's something about hitting -- you lose the confidence, and it's not good. It's a confidence game. I can't sit here and say, 'This guy can't do it.' I will come to Juan's defense. He's still working. It's not like [he's saying], 'What do I care?' He's bothered by this."