Centre-half Dias, 22, is six years younger than Koulibaly, who turns 28 next month, and would cost considerably less.
Benfica rejected £30m from French club Lyon 12 months ago but will cash in on Dias this summer if they receive around £60m.
That represents relatively good value for United, who have armed new boss Ole Gunnar Solskjaer with a minimum of £250m in the current window.