Yes Foukes batting has been far from impressive in the championship this season, the championship for the most part takes place at the start and end of the county season. It is almost the outside wrapper for the glut of white ball cricket that encapsulates most of the season and therefore tends to give green result seaming wickets suitable for the 70 mph dibbly dobbly seamers that move the ball a bit in the air and off the seam and that predominate in the county game.
Foukes has never had any real pretentions as a batter on English wickets, or for that matter on most wickets outside the subcontinent, Sri Lanka however provide wickets that are spin friendly, although sometimes slow, turners. There is no pace, no seam movement, no movement through the air, no intimidation factor and no fear, being able to bat well on such a wicket hardly makes you a good batter on normal domestic wickets with only Taunton coming close.
We can judge that his run of domestic low scores won't give Foukes any real confidence when going into bat but it's to be hoped the special circumstances that is Sri Lanka and memories of having batted exceptionally well there before on his last visit, will still serve him well if called upon.
Tbh because of the above reasoning it's unlikely Foukes would be there for longer than Sri Lanka because of his inability to bat in English (or any non subcontinental conditions he doesn't cope well with the moving ball. It would therefore badly affect the balance of the side if the keeper is a non batter. I see Buttler and Bairstow battling for the position thos summer as the balance of the side is better with a batsman keeper and there isn't too much standing up to spin in England.