The only constants in the Cubs' ever-evolving bullpen this season have been left-hander James Russell and righty Hector Rondon.
Russell is completing his fourth season in the big leagues, while Rondon was taken by the Cubs from Cleveland last December in the Rule 5 draft. Players taken in the Rule 5 must remain on the major-league roster all season or risk being lost.
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The 25-year-old Rondon has held his own and never had to be "hidden" in the bullpen, as many Rule 5 picks are.
"As the season's progressed, he's just gotten better and better," manager Dale Sveum said. "The velocity's gotten better. Obviously, he's turned himself into a power arm. His last part of development has been developing a slider. The whole package, you see the confidence in him.
"He's not 'The Rule 5 guy' anymore. He's actually a guy you can use, and you feel comfortable using him. He's got a little more of an attitude now than he had earlier. He's a guy coming into spring training with that kind of power arm, you've got to take a serious, serious look at."
Rondon entered Monday night's game against the Pirates 2-1 with a 5.05 ERA and a WHIP of 1.47. In 512⁄3 innings, he had given up 52 hits while walking 24 and striking out 42.
Sveum said he told Rondon one thing.
"You got to disregard your earned run average," the manager said. "You've proven to us, and I'm really as proud of him as anybody on the team this year the way he's handled everything, back-to-back days, 2 innings, 40-pitches in 2-inning outings and the whole deal and responded really, really well.
"One thing I'd like to do this last week is get him two days in a row to see what the velocity is like."
Under the knife:
The Cubs said catcher Welington Castillo underwent arthroscopic surgery Monday on his right knee and that the procedure went as expected.
Castillo pulled up lame last Thursday while running out a groundball in Milwaukee. The Cubs are hoping for a 4-6-week recovery period, with Castillo being fine well in advance of spring training.