The righty was claimed off waivers by the Cubs on May 3. Varvaro was also designated for assignment by the Cubs and then outrighted to their Triple-A roster.
The Red Sox stated the following in a release: "As a result of his medical condition on the date of his outright assignment to the Cubs, the severity of which was unknown to the Red Sox at the time of the assignment, both clubs agreed that it would be appropriate to return Varvaro to the Red Sox for placement on the disabled list in accordance with the Major League rules."
Varvaro pitched in nine games for the Red Sox this season, posting a 4.09 ERA.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.