The Twins formally made the transaction on Friday morning, activating the four-time All-Star and placing outfielder Jason Repko on the 15-day disabled list with left shoulder bursitis, a move that is retroactive to Thursday.
Morneau, who underwent surgery to remove a herniated disk fragment in his neck on June 29, hadn't been expected to rejoin the team until Monday at the earliest.
But the slugger looked impressive in his first six games with Triple-A Rochester, hitting .423 with a home run, four doubles and eight RBIs after going 2-for-4 with a homer and three RBIs on Wednesday. He went 0-for-6 in his seventh game on Thursday.
"He's killing the ball," Gardenhire said Wednesday. "He's swinging really good, and feels great. He basically said, "Get me out of here, please. It's really hot in Durham.'"
Morneau was originally placed on the disabled list, retroactive to June 10, with a sprained left wrist, before electing to undergo neck surgery later in the month. He was hitting .225 with a .619 OPS, along with four homers and 21 RBIs in 55 games before hitting the DL.
He was bothered by problems with his neck and left wrist all season before his trip to the disabled list.
It's been a tough year and a half for the first baseman, as he missed the second half of last year with a concussion suffered on July 7 in Toronto. He was putting up MVP-type numbers before the season-ending injury, as he was hitting .345 with a 1.055 OPS and 18 homers in 81 games.
Gardenhire said he's hopeful that now that Morneau is fully healthy again, he can get back to his old ways.
"If we get the guy back who is driving the ball the other way now, and the guy who we've seen hit and be healthy, then I think it'll be real entertaining to watch him," Gardenhire said. "We've seen how he can carry a ball club. If we can get him back and keep him on the field, it will be really entertaining not only for the rest of this year, but maybe having a guy healthy for the start of next year, and we'll go from there."