I am writing a document in XeLaTeX using Libertine and Neo Euler as fonts:
\usepackage[math-style=upright]{unicode-math}
\setmainfont{Linux Libertine G}
\setsansfont[Scale=MatchLowercase]{Liberation Sans}
\setmathfont[Scale=MatchLowercase]{Neo Euler}
Now when I use operators like \log or \cos, XeLaTeX typesets them with Neo Euler instead of Libertine (because of the option math-style=upright, required for Neo Euler). How can I keep Libertine, or set an other font (for example Liberation Sans, my sans serif font), as the font for operator names?

\partial, surely, because otherwise no symbol appears; I can see no difference with\nabla. – egreg Apr 25 '14 at 15:05