The Yankees have made their move for Chase Headley, finally signing the third baseman to a four-year, ~$50 million deal. Not only does it fill the team's need at third base, but it actually offsets the entire roster and makes it a much stronger unit as players shift to new roles in the wake of the signing. Chase Headley doesn't just affect third base, but also most of the infield and even some of the outfield as well.
By bringing Headley back to man third base, it means that Martin Prado doesn't have to. Prado's value is in his versatility, so instead of needing to use him at the hot corner and finding someone else to play second base, they can just make Prado the second baseman to start the season. While fans of Rob Refsnyder or Jose Pirela will be disappointed, you have to understand that the Yankees were not going to start the year with either of them as the only options at second. You might say the Yankees don't give young players a chance, but it's really just that they don't take chances like that. There's a good chance Refsnyder could be bad, so by making Prado the second baseman, they don't have to take a chance in Refsnyder. The move simultaneously secures a mature double-play partner for Didi Gregorius and ensures at least league-average defense at second without having to Refsnyder's defense right out of the gate.
This doesn't even shut the door on Refsnyder, though. There is still one more roster spot that Refsnyder and Pirela will compete over in spring training. If he continues to rake in Triple-A, the Yankees can always promote Refsnyder at some point and move Prado to right field. By that time it's likely that one of Carlos Beltran or Alex Rodriguez will be hurt and the Yankees will either have, or can make, an opening.
As for A-Rod, he is no longer even the backup third baseman at this point. The Yankees were never going to let him start the season at third, but now with Headley in there, Prado has become the real backup. A-Rod will likely be the DH on Opening Day, a duty he will share with Beltran and the other veterans. He will also likely serve as the backup first baseman and maybe occasional third baseman. The former All-Star, MVP superstar will now be the highest paid utility player in baseball.
At the end of the day, this move improves the infield significantly over last year's edition. Having Chase Headley, Didi Gregorius, Martin Prado, and Mark Teixeira is much better than the Kelly Johnson, Derek Jeter, Brian Roberts, Tex garbage infield they went with last season. Prado even doubles as another backup outfielder, possibly even pushing Chris Young to fifth outfielder status. This is how far-reaching the Headley signing is, though it's mostly thanks to Martin Prado.