Lopez, who signed a Minor League deal in January, appeared in six Spring Training games, including four starts. He will be used as the long man but in other ways as well, Cubs manager Dale Sveum said.
"Whenever you have a guy with a slider, the way he used it this spring, I think he's feeling it," Sveum said Saturday. "The flexibility [he provides] is great. He can go long, and you can stack three right-handers up in an inning and bring him in.
"He's a veteran guy who knows how to pitch, and I think with his slider being back where he wants it to be, he becomes more versatile through a whole lineup and through a game when you can use him," Sveum said.
With the switch, the Cubs now have a 12-man pitching staff.
The Cubs claimed Valbuena off waivers from the Blue Jays and he was on the Opening Day roster as the team went with 14 position players and 11 pitchers for one day.
Valbuena, 26, has a career .226 average in the big leagues with the Mariners and Indians over parts of the last four seasons. He was traded from Cleveland to Toronto last November and was in the Blue Jays camp this spring, where he batted .163 in 24 games.