-- A day after losing to one former teammate, Milwaukee Brewers right-hander Tomo Ohka, the Washington Nationals enter a series in which they could face another former teammate, Colorado's Zach Day, more than once.

Day, who was traded on Wednesday to the Rockies for outfielder Preston Wilson, is in Colorado's bullpen for now, and he made his first appearance for his new club Saturday, allowing an unearned run on a hit and a walk in one inning of work in a loss to Cincinnati.

Nationals Manager Frank Robinson, with whom Day had issues during the first part of this season, said Sunday that circumstances worked against Day more than anything else.

"Number one, I just think he was caught up in a situation where we were trying to be the best we could be -- not that we weren't trying to develop some of the young guys," Robinson said. "I think that's what he was caught up in to the point where we couldn't give him the time that he needed to develop at this level. . . . I think, eventually, that had an effect on him mentally, and I think it affected his physical abilities to pitch at this level here."

Robinson said Day, who went 19-21 in parts of four seasons with the Nationals-Expos, still has the ability to succeed, and he believes Colorado could be a good fit. But because he was toggling between the rotation and the bullpen for the Nationals, for whom he posted a 6.75 ERA this year, he couldn't settle in.

"Each time he went out there, he probably felt the pressure of having to be perfect," Robinson said.

Castilla Hurting

Third baseman Vinny Castilla hit two doubles in Saturday night's win, the first time he had more than one extra-base hit in a game since April 16. But Sunday, his chronically bad left knee caused him as much pain as it had all season, and Castilla said he would consult with team doctors before deciding whether to play Monday.

"Today was the worst," Castilla said. "It's been bothering me when I hit. When I put my left foot down and drive from there, my body's still adjusting to the pain, and I'm pretty much using my upper body to hit."

Castilla, who hit 35 homers for the Rockies last year, has six this season.

Zach Day, traded for Preston Wilson, is now in Colorado's bullpen. Nats have a three-game series with the Rockies.