Russell, a converted reliever, may not be a starter for the long term. But, with no one else knocking down the door in the Minors, it appears he'll be in the big league rotation until Cashner can return from a strained right rotator cuff.
"There's a spot available, and we think 'Russ' is our best option," manager Mike Quade said. "If he becomes the second best option at some point, then you make a change. A guy with Major League experience who will do anything you ask who throws strikes, I don't think we have anybody on this staff who we want to test in that role, and nobody in the Minor Leagues is seasoned enough and ready for that. It's an evolving thing."
The Cubs and Rockies will play the middle game of three at Wrigley Field.
Rockies left-hander Jorge De La Rosa is set for his fifth start of the year after turning in his best performance thus far. He allowed two runs on four hits and two walks to the Giants on Wednesday in a 10-2 win.
"He's got great stuff," San Francisco's Aaron Rowand said. "He'll lull you to sleep because he's really slow and deliberate, then pop a 96-mph fastball on your hands."
The Rockies (15-7) are four games ahead in the National League West.
The Cubs (10-12) have yet to get much traction either way this season: on a two-game losing streak currently, they're the only Major League club that's yet to win or lose more than two games consecutively this season.
"We're a good ballclub," said the Cubs' Matt Garza, the losing pitcher in a 5-3 Rockies victory Monday. "We fight and we keep fighting. We're doing all this with practically three starters. We're doing a great job playing at home in these type of conditions and just battling day in and day out. There's no doubt in my mind that we're going to go on a run, and it's going to be fun to watch."
Cubs: Aramis likely to return Tuesday
Aramis Ramirez rested on Monday but should be back in the lineup Tuesday. Jeff Baker started in his place and went 3-for-4 with a run scored.
"Ramy needs a day -- I think he needs a day," Quade said before Monday's game. "It's a good thing when you have a guy playing so well like [Baker] is, it makes it a lot easier."
Starlin Castro committed three errors in the second inning on Monday night, leading to three unearned runs for the Rockies. Garza's throwing miscue was the fourth Cubs error and led to the go-ahead run for Colorado.
Rockies: Iannetta working on flexibility
After two games on the bench because of back pain, catcher Chris Iannetta was back in the lineup Monday and went 0-for-4 with two strikeouts. He sticks to a postgame workout plan, but not one that places strength above all.
"For me it's more stretching, to make sure I keep stretching, and I have been," Iannetta said. "This is something that happened [the injury late in the season]. But I think if it was later in the season I'd have kept playing. It was a situation where we had an opportunity to give it a couple days' rest, and we took advantage of it."
Seth Smith and Dexter Fowler have 17 combined doubles this season, tied for first in the Majors with Alex Gordon-Melky Cabrera (KC) and Michael Young-Mitch Moreland (TEX).