Since most leagues use an X games at position last year or designation by the team as his primary position for this year as an elig rule, I would think Boone will only qualify at 3b in the majority of leagues. He did not play last year and 3b will be his primary position in 05 according to the Tribe.
The question I have is whether his knee is fully recovered to extent that he will bring home SBs. If not, there are a lot of guys at 2B that can be had for really cheap who will get you 10+ HRs, 15+ SBs and will likely have a better average than Boone--Spivey, Matsui and Jimenez come to mind. In a roto league, steals do much more for you in the standings than HRs, so if Boone is not running 100% I think there are better options than his .260 BA and 20 HRs.
Yahoo has a 15 game min rule in the prior year. I do not know if they will carrying elig from 2 years ago forward. I susppect not, but since they are not up for 2005 yet, there is no way to know for sure.