Until Zimmerman returns, Espinosa has got to play. Rendon doesn't look ready, and it would be unfair to expect that: he had one month of pro ball at AA last year, and about two weeks at AA this month. Spring shows that Rendon has a nice swing, but not that he can hit major league pitchers, yet.
When Zimmerman returns, Espi should sit and Lombo should play 2B. If Zimmerman can't return, then the Nats have a hole at the one position without depth. (And I'm watching Span's average angle sharply downward. Not good.)