Last season Tunsil had a 70.6 overall grade from PFF when he played LG, so his grade at LT isn't significantly down and is roughly the league average (last season was 70), and it is an improvement over Branden Albert's PFF grade from last season (42.2). Hopefully Tunsil can kick on at LT and get his grade in the same region as James, at that point we will have two franchise tackles.
In the 2018 draft we certainly need to look at going O-Line early. Someone like Quenton Nelson from Notre Dame would be ideal, or we could look at his Notre Dame teammate Mike McGlinchey and shift Tunsil back inside to LG if we feel he's better at guard. Though in all likelihood I would say drafting someone like Nelson would be the preferred option and then hopefully we find someone else to fill the other guard spot and hope for a Pouncey revival, or just move on from him as well because the interior of the line is worrying looking at those grades and arguably none of them deserve to be retained for next season.
The guards were the problem last year? Not Tunsil, of course.