The Algeria international trained on Friday having not done so since the Foxes rejected a £60m offer from Manchester City for the player on January transfer deadline day.
Although the 26-year-old came under criticism from some fans and pundits during his absence from training, Simpson has claimed that the squad has now drawn a line under the matter.
"He came in and spoke to the lads on our own in the dressing room and that's it, we move on," Simpson told Sky Sports News.
"[On Saturday], in flashes, he has shown why he is a good player and obviously he is back in the team and we are all delighted."
Leicester play Sheffield United in the fifth round of the FA Cup on Friday night before returning to Premier League action against Stoke City on February 24.