Hughes will be examined in St. Louis on Monday by specialist Dr. William Thompson in an effort to determine the cause of his season-long decrease in velocity.
The Yankees believe Hughes may be affected by thoratic outlet syndrome (TOS) and Thomspon is considered an expert on the condition, which affects circulation.
Manager Joe Girardi said on Sunday that he is concerned about what the tests may reveal.
"I think that all of us have been somewhat curious to [find out] what exactly is going on," the manager said.
Girardi anticipated that the results of the exam would be available on Tuesday.
The Yankees placed Hughes on the disabled list on April 15 with what the team called "dead arm." Hughes has undergone a myriad of tests since going on the DL. None have provided a conclusive answer about his arm issues.
General manager Brian Cashman said Hughes showed a "slight positive" on a test last week. That led the Yankees to believe TOS might be the cause of Hughes' dead-arm problems. Hughes, though, has not been diagnosed with the condition, which is why he is being sent to the specialist.
Hughes went 0-1 with a 13.94 ERA in three starts in 2011. His fastball has lacked velocity all year. He was throwing pitches at an average of 89.3 mph this season, according to FanGraphs.com. His average fastball was 92.6 mph in 2010 and 93.8 in 2009, according to the website.
The Yankees initially hoped that Hughes would be able to regain velocity through exercise and long-toss drills. But last Monday, Hughes stopped a bullpen session early because of discomfort. He visited with the Yankees team physician, Dr. Chris Ahmad, that night and underwent a vascular test and two MRIs later in the week, leading the Yankees to believe Hughes may be suffering from TOS.
Several pitchers have dealt with the condition, including Kenny Rogers, Jeremy Bonderman and Aaron Cook. Treatments can range from exercise programs to surgery, and the time it takes to recover can range from a few weeks to an entire season.
In the case of Rogers and Cook, they came back to pitch as effectively as ever. Bonderman never quite regained the form he had earlier in his career.
"Anytime a guy is having an exam, it's a concern," Girardi said. "You just want to see the young man be OK and get back on the field, that's the important thing."
The 24-year-old Hughes was counted on to anchor the starting rotation along with CC Sabathia and A.J. Burnett, after going 18-8 with a 4.19 ERA last year. His performance dipped after he was picked to his first All-Star team, though, and that carried over into this season.
He was 0-1 with a 13.94 ERA after three starts.
Garcia has made three starts after an early relief appearance, and has allowed only four runs in 18.0 innings, though he was hit hard his last time out. Colon is 2-1 with a 2.77 ERA, and allowed only one run in eight innings against the White Sox on Wednesday night.
No one knows how long Colon can last, but he's been a huge upgrade from Hughes thus far.
Hughes, a California native, is just one of three Yankees in the live ball era to allow at least five runs and pitch fewer than five innings in each of his first three starts this season. Even more alarming, Hughes had given up four home runs and struck out just three batters in 10 1/3 innings in 2011.
The Yankees haven't ruled out the possibility that Hughes' arm strength has been affected by the amount of innings he threw last season. The team placed Hughes on a strict innings limit in 2010, skipping his starts periodically to keep his count down.
He finished with 176 1/3 innings pitched. His previous major league high was 86 innings pitched (2009).
Regardless of when Hughes returns to the mound or what his tests reveal, Yankees pitching coach Larry Rothschild will be concerned until he sees Hughes' fastball return to its normal velocity.
"It's been a while now since he's pitched. Whatever the exam says there's still going be concern," Rothschild said. "... Until you see him throw the ball like he's capable you're going to be reticent."
As will Girardi. More than anything else, the Yankees manager is looking forward to finding an answer on Hughes, who is expected to be a part of the team's rotation for years to come.
"You talk about a number of different things," Girardi said. "You talk about the innings last year, Phil in the past has been a slow starter with his velocity, and that's something that's continued to be a red flag. But there is some curiosity."
Ian Begley is a regular contributor to ESPNNewYork.com. Information from The Associated Press was used in this report.