The main problem I see is unfortunately not fixable.
Lampard can definitely improve the structure in midfield and how we defend as a unit, but the biggest issue that even Sarri couldn't fix, is the lack of end product.
Willian averages 5 league goals a season
Giroud isn't prolific and his attributes don't make him a good fit for Lampard
Tammy is unproven.
Pulisic is unproven.
Pedro is over the hill, and hardly a chance creator.
CHO, as much as we hype him, is unproven.
Mount and Barkley are probably the most creative. The former unproven and the latter notoriously inconsistent.
The fact of the matter is, the real chances we do create, is from forcing the opposition to make a mistake. When our press slows down, the chances we do create dry up as there is little to no creativity in the team. The chances we do get, we fail to convert. So it breaks down to whether or not we kill the game early on, or allow the opposition back in it.
Let's be real here, there's a reason we saw little to no comebacks last season.