Hunter, 29, had a 3.63 ERA in 39 games this season, striking out 32 and walking 11 over 44 2/3 innings. Cubs manager Joe Maddon said the hard-throwing right-hander could be used in late-inning situations, which includes nearly everyone in the bullpen.
Hunter was the Rangers' first-round pick in 2007 before he was dealt to the Orioles in July 2011 along with Chris Davis for Koji Uehara. Hunter also can help the Cubs next year; he's a free agent after the 2016 season.
Since moving into a full-time bullpen role with the Orioles in 2013, Hunter has gone 11-9 with 39 holds, 15 saves and a 3.05 ERA in 167 relief outings, limiting opponents to a .276 on-base percentage and .357 slugging percentage. He has walked 37 batters in 191 2/3 innings and posted a 1.95 ERA in 21 outings against the National League the last three years.
"In Texas, he had success as a starter and then struggled, and they put him in the 'pen," Arrieta said. "He's really been able to flourish over a consistent period of time with his stuff. He's embraced the situation."
The Cubs have been looking for pitching help in their bid to secure a playoff spot. Chicago began the day third in the NL Wild Card race behind the Giants and Pirates. Did they do enough?
"We definitely have enough," Arrieta said. "We just need the guys we have to step up. That's the same story for a lot of teams. The Blue Jays stand out as the team that made the most noise. I think some subtle additions can help us in a big way.
"As a staff, both as starters and relievers, we can shut teams out, we can keep it to two, three runs a game," Arrieta said. "That's good for our lineup to not feel pressured to score a ton of runs. We just need them to score a few."
Lake, 25, was batting .315 at Triple-A Iowa with seven home runs, 10 doubles and 31 RBIs. He was called up to the big leagues this season for 21 games and batted .224 with one homer. The outfielder made his Major League debut in 2013.
To make room on the Cubs' 40-man roster, right-handed pitcher Ben Rowen was designated for assignment. Rowen, 26, was selected from Triple-A Iowa on Wednesday but did not appear in a game for the Cubs.