Infielder Corban Joseph was recalled from Triple-A Scranton/Wilkes-Barre to replace Youkilis on the active roster, and the Yankees are hopeful that Youkilis will be able to return in the minimum of 15 days.
"He's got an irritated back, basically," manager Joe Girardi said. "Hopefully, it's only the two weeks that we get him back and get him playing at the level that he was playing at. Short term, I'm a little concerned about it, but it's something that's going to go away."
Youkilis had taken six days off with the back issue before telling Girardi and the coaching staff that he was ready to play, going hitless in the Yanks' 5-4 win over Toronto.
He felt more discomfort on Sunday, ultimately missing eight of nine games before being placed on the DL on Tuesday. Cashman said that the Yankees had been thorough in deciding to allow Youkilis to play, but he still wished he could have a do-over.
"You do the fire drill: go through it [with the] trainers, manager, coaches, player, doctor, front office -- everybody is involved," Cashman said. "If it works, you don't look back. If it doesn't work, you have regret. If we could do it all over again, we would not have played him Saturday."
An MRI examination taken on Youkilis' back was negative for further damage, but he was given an epidural injection on Tuesday to alleviate the discomfort. Cashman said that Youkilis was told not to report to Yankee Stadium for the club's game against the Astros.
Girardi said that Youkilis will soon begin back stabilization exercises and could resume baseball activities within seven to 10 days, and that he did not suspect any reason to doubt Youkilis' insistence that he was ready to play.
"Usually when a player tells you he's ready to go and he's an experienced guy, you feel pretty good about his answer," Girardi said. "If it's a young kid, you might think, 'Is he trying to tell me something?' or 'He doesn't want me to think that he's soft' or 'If I don't play, I'm going to lose my job.' A veteran player, you don't worry too much about those things."
But Girardi has learned a lesson in communicating with Youkilis and said that under a similar future situation, "I might ask him six more times."
Cashman said that he believes Youkilis aggravated his back on a play in Saturday's game where he had to slide into first base on a putout, because pitcher CC Sabathia was unable to cover the base in time.
"I knew when CC fell off the mound the way he did and didn't get over, [Youkilis] had to slide into first base," Cashman said. "His facial expression from that point on, I was like, 'We're going to have a problem.' And we did."
The 34-year-old Youkilis signed a one-year, $12 million contract with the Yankees and was expected to hold down third base in the absence of Alex Rodriguez, who is not expected to return until after the All-Star break.
Youkilis was batting .266 with two homers and seven RBIs in 17 games, and he has dealt with back issues in the past, missing 14 games with back stiffness in 2011 and another 22 games last season with back tightness while with the Red Sox.
"Even not knowing Youk very well, but knowing Youk from afar, he's someone that's just a tough guy," Cashman said. "It is what it is. You can't turn it back. But if you're asking me if we could do it all over again, would we do it differently? The answer is, of course, yes."
Joseph, 24, was recalled from Triple-A Scranton/Wilkes-Barre, where he was batting .273 with four home runs and nine RBIs in 22 games.
A fourth-round selection by the Yankees in the 2008 First-Year Player Draft, Joseph has played mostly second base in the Minors, but he could also be used at third base or first base. Cashman said that Joseph was promoted because he is on the 40-man roster.
"Ideally, is he the perfect fit? No, because he's a left-handed bat," Cashman said. "But he's the infielder who is on the 40. He can swing the bat and all that stuff."