I think theyve always had that tile problem during lift off and its not much of a risk only if a big amount of tile have been hit during lift off in the case of the shuttle that desintegrated during re-entry. Nasa postponed the coming back for a day or two so the astronauts could do some more check ups on the shuttle's body it seems they found a miniscule hole in one of the tiles that may cause the other tiles to heat up beyond their heating point and causing the ship to blow up. Hopefully theyll have a succesful re-entry, best of luck to them