"He's going to have to pick it up here in the next few games," Melvin said. "I want to see results."
The 27-year-old showed plenty of results last season as he compiled a 1.88 ERA in 40 games at Double-A, along with a 0.45 mark in Triple-A before earning a September callup. In nine games (6 2/3 innings) with the D-backs he did not allow a run and held lefties to a .111 batting average.
This spring, though, it's been a struggle for Slaten, who was struck on his forearm by a batted ball and has battled some back stiffness this month. He put together a nice outing on Wednesday against the Angels and seemed to have turned the corner, but after a strong first frame Saturday, he was unable to get out of his second inning of work.
"More struggle than good," Melvin said in summing up Slaten's spring.
Slaten faced 95 left-handers last season and struck out 42 of them. The presence of an effective left-handed specialist is especially important in the National League West with lefty hitters like Barry Bonds and Todd Helton, among others.
The fact that he would be used in crucial situations late in a game makes Melvin even more reticent to keep him on the roster if he's not pitching well.
"That's a big spot," Melvin said. "It's a big spot. So I want him to be confident. We want to be assured he's ready to go in those situations. He knows going in he's got to go out and produce. All these guys have to go out there and produce. No one has any job completely secured. That's the way I want them playing, and that's the way I want them pitching."
Slaten likely has three more appearances this spring, which leaves him with plenty of opportunity to still secure a spot. The D-backs have limited left-handed options if Slaten is not on the roster.
The club could use Dana Eveland in the lefty specialist role, but if he makes the team it would seem he would be a better fit as a starter for a couple of reasons. First, he hasn't had a lot of experience in a specialist's role, and second, if he's used in a relief role the first few weeks of the season and throws only a handful of innings, it would be hard to get him stretched back out if they wanted to use him as a starter.
"His stuff is conducive to getting anybody out," Melvin said of Eveland. "It's just at times when he's had some issues, it's been with walks and command. In a situational role it's certainly a guy you want to have good command. I wouldn't say that that's one [role] that we're grooming him for right now."
So is it possible they enter the year with no lefties in the bullpen, a situation they had for much of last season?
"We've shown in the past, too, that we can go with righties," Melvin said.
Brandon Lyon, Brandon Medders and Tony Pena are all right-handed relievers who have shown they can get left-handers out, and all three figure to pitch late in games for Arizona this year.
"All of them have the weapons to be successful against left-handers," Melvin said. "When you pitch deep in games, especially if we're limited in what we have down there as far as left-handers, you've got to be able to get them both out. We don't want you looking over your shoulder in a situation where you have to get a left-hander out. You've got to want it and expect to be out there."