Guerra allowed a two-run walk-off double to pinch-hitter George Kottaras with one out as Milwaukee rallied in the bottom of the ninth for a 5-4 win.
"I guess it's the personality," Mattingly said. "He's a carefree guy, he gets on the mound and he lets it go. He'll be mad about the game last night, then it's over."
Guerra blew only two of his 23 save opportunities last year after inheriting the closer job from the injured Jonathan Broxton. Although Kenley Jansen has dominating strikeout totals, Mattingly has insisted that the more seasoned Guerra remains his closer, with Jansen setting up in the eighth inning.
"The ninth inning is always nerve-wracking," said Mattingly. "But Javy doesn't get rattled. Last night, he just didn't get the pitch where he wanted. But a one-run lead in the ninth inning? I'll take that situation every day."