I havent seen much of Johnson other than Vitali fight and Prizefighter however i can see him giving Fury some trouble. Against Vitali he looked slick but Vitali was too smart and wouldnt commit and leave gaps to exploit. Fury is no where near the boxer Vitali is, at least not from what we've seen. He leaves openings he attacks and loses his composure which could play right into the hands of the counter punching Johnson.
BUT Fury is massive. 2-3 inches taller than Vitali, who dwarfed Johnson, and is coming in proper shape for the first time that ive seen. Johnson also is a late replacement and may not be in perfect condition. I predict both having alot of trouble landing on each other early on, then either Johnson loses steam and Fury takes over. Or Johnson starts to frustrate and catch Fury with counters.
A problem i see for Johnson also is that with his style he will struggle to get a decision, especially in Ireland where Fury has a lot of support.
Like i say havent seen much of Johnson but from ive seen, and heard from people i respect, hes gonna be a very live opponent for Tyson. Definitely excited for this fight!