Cotts had given up eight runs on nine hits and seven walks over nine innings in seven games this spring, while Hill was batting .083 in nine games.
"Hill does a nice job behind the plate," Cubs manager Lou Piniella said. "My message to him was he just has to hit a little more. Cotts, I thought he was throwing the ball better as Spring Training progressed. Hopefully, he'll go down to Iowa and pitch well and help us when we need another pitcher."
The Cubs now have left-handers Scott Eyre, Sean Marshall and Carmen Pignatiello in camp, but most likely will break camp with only Eyre in the bullpen. Marshall started on Friday against the Colorado Rockies.
"I think the only other lefty here who has a chance of making this team is Marshall," Piniella said when asked about Pignatiello, who has not given up a run in 5 1/3 innings. "I'm not talking about in the rotation, but in the bullpen, too."
Bob Howry was more effective against left-handed hitters (.192) than right-handers (.283) last season, while Carlos Marmol has held lefties to a .209 average compared to .146 against right-handed hitters. Right-handed hitters were batting .233 against Michael Wuertz compared to .238 by lefties.
The Cubs' spring roster now is 36 players, and includes 18 pitchers, two catchers, 10 infielders and six outfielders.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.