The Nationals did well to retain Stephen Strasburg via free agency and have been aggressive in bringing in ancillary parts around the infield, but in losing Rendon, they’ll need to do more in the infield. Cabrera is a nice spare part to bring back, but the infield remains incomplete at this point. 
Cabrera can handle second or third base adequately, in theory, but he’s lacking range at this point in his career. 
The question moving forward is if the Nationals are truly going to replace Rendon. Neither Castro nor Cabrera is a replacement of a superstar. Donaldson in free agency could be. If the Nationals don’t do that, would they pony up for Kris Bryant via trade? He’s known to be on the block. 
Cabrera, 34, won a ring with the Nationals last season. He was signed by the Rangers in January but then released on August 3. Three days later, the Nationals signed him as a free agent. In 38 games down the stretch, Cabrera slashed .323/.404/.565 with six homers and 40 RBI while walking more times than he struck out. He wasn’t quite as productive in the postseason, hitting .233/.250/.267.  The Nationals likely aren’t done adding this offseason — and Josh Donaldson to replace Anthony Rendon at third base would be the big fish — but they have given themselves depth around the infield. The only sure thing right now is Trea Turner at shortstop. 
As things currently stand, Howie Kendrick, Starlin Castro and Cabrera fill in first base, second base and third base in some fashion. It would likely fall out with Kendrick at first, Castro at second and Cabrera at third. Wilmer Difo and Adrian Sanchez are also options, while Carter Kieboom still has some prospect shine and could be a fit at some point. 
The Nationals have agreed to terms with infielder Asdrubal Cabrera on a one-year, .5 million deal, per multiple reports (Ken Rosenthal of The Athletic among them).