The Wales international has taken the decision to further his development by experiencing playing abroad.
Levitt, who made his senior United debut in the Europa League clash with Astana last season, returned from his temporary spell at Charlton Athletic in January after United activated his recall clause.
The midfielder could make his Istra 1961 debut in their Croatian Cup round-of-16 match against HNK Sibenik on Wednesday.
United currently have nine Academy players out on loan from the Under-23s group with the players experiencing a wide range of opportunities which will stand them in great stead when they return to the club.