It's not uncommon for the Spurs to look for reinforcements in the European market. That's how they got Gary Neal and Aron Baynes, after all. What made the signing of Boban Marjanovic a surprise was that the front office was filling a position of need with an unproven player at the NBA level. With Tiago Splitter and Baynes gone, Marjanovic is the only true center on the roster.
The concerns about his addition have little to do with talent or credentials. Marjanovic was the breakout star of the Euroleague last season, a 7'3" behemoth that was among the best post players and finishers in general in the competition.