Lewis is playing pretty much as a last resort. Now that Nunes has shown signs of being an OK-ish RB, Lewis is playing less.
And even if you disagree still - watch the next game Lewis plays - closely focusing on Lewis. Watch his movement when we have the ball. Is it good? Or is he often unaware that he's covered or that he's in a line-of-three and thus should move, but does not?
Watch Lewis on defense. Is he good? Does he read the game? Does he charge forward into an attack when he has ample coverage behind him? Or instead, does he simply back up?
What about defending crosses? Is Lewis good at that? Watch the next game he's in and decide.
Or on set plays when on defense. When he's picked by an opponent - obviously blocking his progress - does he figure out what's going on? Does he fight the pick moving to cover an attacker coming in for a pass, currently not covered b/c the attacker is several yards outside the box?
Progressive forward passing. Is Lewis good at that? Or is it mostly safe non-threatening passes?
What about in front of goal? Does Lewis deliver good passes/crosses or shots on goal?
If you somehow think that Lewis is a good or up-and-coming player with potential - I honestly can't believe that you've taken time to watch his play.