"It's not how you draw it up, there's no doubt about that," Cashman said. "Kevin is a [great] player when healthy, he just hasn't been in a position to show what he's capable of in-season because of the back. He looked great in the spring, and we had high hopes. He obviously did so much for Boston over the years."
Youkilis, 34, played in just 28 games this season after signing a one-year, $12 million contract in January, and he was batting .219 with two home runs and eight RBIs. His current DL stint is the second of the year, as he missed 30 games with a sprained lumbar beginning in late April.
The Yankees were aware of Youkilis' history of back trouble, but he passed all of his physicals before the season and was being counted on to continue excelling against left-handed pitching with Alex Rodriguez not expected to return until after the All-Star break.
"With the Alex injury, he was by far the best option, as long as he was healthy," Cashman said. "Unfortunately, his health has taken a turn for the worse. He's not available to us, [we've] got to get him fixed; [there's] nothing we can do about it."
The Yankees can continue to use David Adams, Reid Brignac and Jayson Nix at third base, and Cashman said that even with Rodriguez apparently on the comeback trail -- he tracked pitches on Tuesday in Tampa, Fla., one small step from taking batting practice -- he will be open to other options to replace Youkilis.
"We're always looking to upgrade if we can regardless of position," he said. "You have to keep in mind, too, that we expect to get Alex back. As of right now, we're going with what we have, and if we ever run into something that makes sense, then we'll look at that. We'll keep going with what we've got."
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. Josh Vitale is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.