An MRI performed Tuesday afternoon on the righthander's upper back showed a bulging disc, and Cashman said Wednesday the pitcher would be sidelined for a "two-week timeout."
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"Hopefully it's nothing more than that," Cashman said.
Hughes, who was injured Monday covering first base during infield drills, said he will be on anti-inflammatories for five days and do "some pool work after that if I'm symptom free."
"Best-case scenario, I pick up a ball in six, seven days," Hughes said.
But the unpredictable nature of back injuries had Cashman and Girardi speaking with plenty of qualifiers.
Catching prospect Austin Romine, to name one example, was shut down early last spring with back soreness that initially was to keep him out a few days. He eventually was diagnosed with a bulging disc in his lower back and didn't return until July.
"I am pleased that he feels a lot better than he did a couple of days ago but you still have to worry about it," Girardi said of Hughes. "The fact that he's made a lot of progress in the last two days makes me feel better but until he's actually going through what he has to go through as an athlete there's going to be some concern."
Cashman said he was encouraged, but only to a point.
"The doctor clearly isn't aggressively treating it and obviously that's a good sign, but there's no guarantees until we get through the two-week process and see where he's at and he's back on the mound," Cashman said. " . . . We won't be out of the woods until we see all that stuff. So I can stand here and say, yeah I'm excited and thankfully it's a low-level situation and blah, blah, blah. But I think we really need to get through this stuff to make sure it responds that way. It's a question mark until we can forget that it ever happened."
Hughes, 26, said the diagnosis was "good news," based on how he felt initially.
"The first day was rough getting out of bed and stuff like that and now I'm moving around a lot better and not feeling it just standing here or anything," Hughes said. "I feel like in three, four days, once I go through these meds, I'll feel even that much better."
Hughes, slated to be the team's fourth starter after going 16-13 with a 4.23 ERA last season, did not use the word concern.
"Just with how I've been progressing the last couple of days, I feel like I'll get through these anti-inflammatories and I'll be symptom free in the next four or five days and we'll get going from there," said Hughes, who heard some good-natured teasing from veterans like Andy Pettitte about being among the first players hurt in camp. "And for me the positive thing is that I felt like I was ahead of the game a little bit, throwing well, pretty progressed in my throwing program. Hopefully this setback doesn't affect me too much."
He added: "It's a setback, but we still have a lot of time to get it right and not push it and make sure I'm 100 percent healthy . . . It can be a recurring situation if I don't take care of it now."
Girardi said he didn't think the opening week of the season for Hughes is in jeopardy yet, but the nature of back injuries gives him pause.
"If you're going to get a break, I would rather it be early in spring training because I think you can make up a little bit of time than later," Girardi said. "But we'll have to see where he's at."