I concur...the mug was produced much, much later than when the George Grooms gopher was used (1940s) . I remember getting a ton of these same mugs, but only Minnesota Viking versions, in the 70s and 80s.
The other odd thing about the logo is the "M" in the background. I don't ever recall the U officially using an "M" where the bottom middle serif comes all the way down to the baseline, but I could be wrong. Let me know otherwise. I know, font/logo geek...right?
Regardless, great find. I would have snatched it up, too!