The Twins were hopeful that Mastroianni would be able to get through waivers, but he was ultimately claimed by the Blue Jays, which was his original organization. Mastroianni was acquired by the Twins in a waiver claim from Toronto in February 2012.
"It's disappointing," Twins assistant general manager Rob Antony said. "He's a good guy and a good insurance player, especially as a guy who can run and play all three outfield positions."
Mastroianni saw his 2013 season marred by a left ankle injury, having injured it in Spring Training on a foul tip before having surgery to repair a bone chip in his ankle in May. He then underwent a second surgery on Oct. 17 to remove the fixation hardware originally placed in his ankle.
Twins manager Ron Gardenhire said he believes Mastroianni is past the ankle injury, and was also disappointed to see him claimed by Toronto.
"We like Mastro an awful lot," Gardenhire said. "He was struggling swinging right now, so we were hoping we could send him down for some consistent at-bats. He was coming back good from that ankle injury. But I'm happy for him. If somebody wants you, that's always good. And we wanted him, too, but it didn't work out."
Mastroianni fell victim to bad timing, as the Twins needed to create a roster spot for Fuld when they claimed him on Sunday.
The Twins had hoped that the paperwork for Jason Bartlett's retirement would go through in time to have an open spot on the roster, but Bartlett's retirement didn't become official until Monday because of the holiday weekend. So the Twins had to designate Mastroianni for assignment to clear a roster spot, even though he still has a year of Minor League options remaining.
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.