"It's just the title of my job," Villanueva said. "We all know what's happening. It's not going to shock anybody -- 'Oh, Arrieta's back.' If it's me that has to go down [to the bullpen] and fill another role in the bullpen, then I'll go. If I haven't complained before, I'm not going to start complaining now."
Last year, it was a similar situation as Villanueva filled in while Matt Garza was rehabbing. Arrieta has been slowed this spring by tightness in his right shoulder. He is scheduled for another simulated game this week, as he continues to make progress.
In 15 starts last season, Villanueva was 1-7 with a 4.50 ERA, and was 6-1 with a 3.03 ERA in 32 relief appearances.
"He's been in that role, he's been a pretty good performer for the Cubbies," Cubs manager Rick Renteria said. "He's done a good job in any situation we've used him."
Villanueva's first start will be April 6 against the Phillies. The Cubs want to keep Opening Day starter Jeff Samardzija on schedule, so he will start Monday and April 5.
Rusin had pitched well except for his last outing against the D-backs on Wednesday, when he gave up nine runs on 10 hits, including four home runs, over 2 2/3 innings.
"He should be proud of how he went about his business," Renteria said of the lefty.
"My initial reaction was Rusin," Villanueva said. "I know it had to be difficult. I spoke to him a little bit. We know he'll be here at some point. It still doesn't make it easier. It came down to the wire. I just found out today."
How is Villaneuva able to switch roles from starter to reliever so smoothly?
"I wasn't blessed with a blazing fastball but I was blessed with a healthy arm," he said. "I'm able to bounce back."
For example, he threw 90 pitches Monday, then two innings on Thursday, and felt good.
"Not a lot of guys can do it," Villanueva said about being able to change roles. "Maybe a lot of guys don't get the opportunity to do it. Maybe I can be one of those people who advocates and fights for the rights of the swing guy."
It wasn't the role he projected himself in.
"When you get up to the big leagues, you're thinking, 'I want to be an All-Star, I want to be a No. 1, No. 2 guy,'" Villanueva said. "The years go by and the stats don't lie. It's what I've done the last three, four years. Thirteen, 15 starts, and 30 [games] in the 'pen. 'Good job.' That's all I want to hear -- 'Hey, Villa, good job.'"
Fans will be happy to know that Villaneuva is keeping his handlebar mustache. He shaved last season, and quickly grew it back.
"The day I after I shaved, I looked in the mirror and I was very upset," he said. "I don't want to feel that way anymore. As of now, it's staying. I wouldn't feel normal [without it]."