No he wouldn't, not all of the time. But we'd have a better option from the bench to replace Eriksen than Sissoko or GKN (both of whom Poch clearly doesn't rate thjat much).
Actually I think not having Lamela hurt us in the cups, part because his motivation is never in question, part because he gets a little more time/space, he tends to run some cup games.
Without Eirksen or having an off Eriksen with a rotated squad often hurts us, Lamela can offset some of that.
And as others have said, given the choice of Sissoko/GKN or Lamela coming off the bench when we need something from game, I know which one I would pick