Team doesn't have to sign anyone. If a player really want to come over right now, but team that drafted him doesn't, all he can do is press his agent to find another team with assets, who likes him. NBA teams lose rights to drafted players, only if they don't play pro basketball for 1 season. Most NBA ready guys from Europe are earning salary comparable to what they would be making in NBA. That's why Pekovic was #31: he had buyout issues at the time of the draft, and it was understood, that by the time he would be ready to come over Pekovic would make much more in Europe, than the salary of a pick in the 20s would offer. 2nd round picks are indeed more preferable as it doesn't lock players into fixed salary scale. However in current CBA this rule was modified, and now in three years after the draft teams can ignore rookie scale for first-rounders, and offer players any amount of money they have available for any free agent, as long as it's more than respective rookie scale. That's why it's now possible for Bulls to lure Mirotic from Spain (he was drafted in 2011). Still if you draft&stash a player with first-rounder, and he becomes really good in two years, you'll likely have to wait another to get him.