He should be NHL ready, but he isn't ready to carry a line as a center without a high-quality winger to help lift the burden. If he continues to struggle they should just put him on Zajac's or Hischier's wing for most of the year. Very few guys are capable of carrying a line as a center as a D+1 rookie. You have examples like McDavid, Matthews, Eichel, and Tavares, but other very highly touted prospects had issues like Stamkos who played center but struggled (and was misused by Melrose), and Seguin who played RW due to going to a deep team. Hischier did well in year 1, but had a very high-quality winger right off the bat who had a long track record of driving play. Others have been passable on the 3rd but are generally significantly bigger than Hughes such as Monahan, Patrick, and Barkov.
Basically, the Devil's have two reasonable choices if these struggles hold on for a couple more games. First, is move him to the wing, or two, put Taylor Hall on his line. I can't see an outcome where they send him down after 9 to protect his ELC. It would reflect poorly on too many people even if it is the right move (and, I don't think we have enough evidence to say he should go to the AHL yet).