I never heard anyone call Spencer the next Mantle, Mass either.
The only one I remember was Bobby Murcer.
His was really a burden because Mantle had just retired and the Yankees sucked and needed a savior. They would always point out the similarities like they were both from Oklahoma, both signed by the same scout, both moved out of the infield because of throwing problems.
Both came up as Shortstops, although Murcer began the 1969 season at 3B but kept throwing the ball in the stands.