"It's a dream come true -- I've dreamed about this as a little kid and it finally happened," said Rusin, who will be officially added to the roster on Tuesday.
The Cubs' fourth-round pick in the 2009 First-Year Player Draft, Rusin was 8-8 with a 4.59 ERA in 24 starts at Triple-A Iowa, striking out 87 over 133 1/3 innings. He got off to a solid start, posting a 2.63 ERA in his first five games, but struggled in June. He's rebounded, and in his last three starts, Rusin has given up four earned runs over 18 2/3 innings.
"I just had to sit down and watch some video and figure some things out, and we did," he said.
His last outing was Wednesday against Tucson, and he threw six scoreless innings, giving up four hits in the win.
"It's no different than pitching in Triple-A," Cubs manager Dale Sveum said. "You still have to make your pitches."
Rusin, 25, will likely be backed up by several of his former Iowa teammates who have already been promoted to the big league team.
"You see people going in and out, and people moving up and down," he said. "It's all about timing, I think. When you get your chance, make the most of it, and that's what I'm going to try to do tomorrow."