Blanco injured his left thumb in Sunday's game against the Phillies. He flew back to Phoenix on Monday for an MRI and to be examined by hand specialist Don Sheridan, but signs pointed to him having a torn ligament, which would likely end his season.
"He just woke up today and was way swollen, and it was way loose," D-backs manager Kirk Gibson said of Blanco. "Henry's thumb was not good today."
Nieves, 34, hit .298 in 16 games with the Rockies before being designated for assignment last week. When they realized Blanco's injury was likely to keep him out of action for a while, the D-backs quickly put in a waiver claim on Nieves.
"The first couple of days I'm going to be lost, but it's going to be good, man," Nieves said. "It's a good team, they're in a great position and I'm going to try to help them any way I can."
Nieves has played in parts of nine big league seasons with the Padres, Yankees, Nationals, Brewers and Rockies.
"It was surprising, because I was having a good year with Colorado," Nieves said of being designated for assignment. "In the beginning, it was frustrating that it was happening to me again now that I'm having a good year, but God is always in control. He's always in control of my career, and if he closes a door somewhere, he's going to open another one. So maybe I thought that was the best for me, but maybe this is the best for me right now, so I'm just happy to be here."
In order to familiarize himself with the D-backs' pitching staff, Nieves plans to catch the starting pitchers' between-starts bullpen sessions as well as catch relievers warming up in the bullpen when he's not playing.
"Every time you get to a new organization, they do things different, but it's the same baseball. And when I go out there, I just want to do what I know how to do," Nieves said. "Obviously I need to learn the pitchers and what they like to throw, and that will come with me coming to the bullpen and talking to them, communication."