According the the all-22 types, Campbell's problems are somewhat more with bull rushes, with losing battles of strength, than with speed rushes. I know he has had problems with both, hence the "somewhat."
Against the speed guys, better technique, quicker recognition skills ,better teamwork with his guard and with the chip-blockers, can compensate for his presumed speed deficiency, but speed is pretty much, I think, a fixed quantity. You can work at it, but you are not going make any dramatic change in that attribute. Strength, which is, we are told, a somewhat greater issue, is very much something you can improve, by hard work in the weight room and in other aspects of training. This seems to me a hopeful thing. No LT is going to be perfect, and if he can improve his performance against strength moves and gain incrementally in other areas, that seems to me a pretty hopeful scenario.
All that said, I assume they will base how they handle Campbell on an empirical basis going forward, and if Campbell proves not to be capable of playing LT at a high enough level, I don't see it as an apocalyptic failure if he ends up as a guard, likely a really good one. Nearly all teams struggle with manning their O-lines. There just aren't all that many 300-pound stellar athletes in the world, and that is what it takes. The NFL is an athletic freak show and a good freak is hard to find.
I think he'll be fine at LT, and if they decide he isn't, that's probably going to be fine as well.
FWIW, I suspect his injury is playing a part in his difficulties as well. It's just hard to trust a knee once it has let you down, and connective tissue takes forever to heal, when it does heal, so I am sure that plays a role.