From the Sporting News, March 29, 1886:
"A Benefit For Tom Sullivan
John T. Magner, the well known ex-professional, is getting up a benefit game for Tom Sullivan, who recently had the fingers of his right hand amputated. It will take place at Union Park some time in the near future, Mr. Lucas having tendered the use of his grounds for that purpose. Mr. Magner says that Ed Cuthbert, Charles Houtz, Harry McCaffery and several other old professionals have promised to assist him in getting up the benefit, which promises to be a veritable bumper."
Note that this Sporting News article states that Sullivan lost the fingers of his right which contradicts Al Spink's claim that Sullivan had both of his hands amputated. Also, they say this happened "recently" which gives further evidence to the idea that Sullivan's injury happened in the winter of 1875/76.