According to the Minneapolis Star-Tribune:
Allison, a rookie receiver, will become the first Vikings player to wear No. 84 in a regular-season game since Randy Moss was traded to Oakland after the 2004 season. Allison made the switch from No. 14 to Moss' old number on Thursday. Allison admitted he was getting heat from teammates about his decision.
I'll say this about Allison. . .for him to make this switch as a rookie shows that, at the very least, he's got juevos grandes. Hopefully he can provide even a fraction of the production on the field.