There should be no obstacles for Donatas Motiejunas to join the national team this summer as far as his contract is concerned. As it was pointed out by Krepsinis.net, the player is under contract with Benetton Treviso.
Motiejunas, who turns 22 in late September, joined Asseco Prokom on loan from Italian side Benetton Treviso. He still has a valid contract with the Serie A side which means that he won't be a free agent before joining Houston Rockets.
Under these circumstances, the 2.13m power forward wouldn't risk losing a contract if he gets injured while playing for the national team.
However, the insurance should become higher once he signs a contract with the Rockets due to a larger salary.