By Ransdell Pierson

The merger could have been a defining triumph for Gonzalez, a quiet and low-profile manager who has been building an ambitious lineup of experimental drugs since his company was spun off in early 2013 from Abbott Laboratories. During his decades at Abbott, Gonzalez seldom emerged from the shadow of blustery Chief Executive Miles White, and the Shire deal gave him a rare burst of limelight.

Investors were given no advance warning on Thursday, when AbbVie recommended that its shareholders reject the Shire transaction, and it agreed to pay Shire a $1.64 billion breakup fee.

"This came as a complete surprise both to investors and to Shire and they have a legitimate gripe about how AbbVie handled the communication," said Sanford Bernstein analyst Ronny Gal. Although it could take AbbVie a long time to regain investor trust, Gal predicted investors will not hold Gonzalez, 60, personally responsible for the collapsed Shire deal because circumstances changed in recent weeks.

By buying Shire and relocating the combined company to Britain, AbbVie hoped to slash its corporate tax rate, in one of the biggest such "tax inversions" attempted by any U.S. company. And Shire's lucrative drugs for rare diseases and attention deficit disorder would have lessened AbbVie's reliance on its arthritis treatment Humira, the world's biggest-selling drug, whose $13 billion in annual revenue comprise almost 60 percent of AbbVie sales.

AbbVie late on Monday said its board pulled out of the deal because of reinterpretations of tax law made on Sept. 22 by the the U.S. Treasury Department, which would have destroyed financial benefits of inversion deals.

"Treasury flipped it on its head, turning a very good deal into a very bad one," Glenn Tilton, an independent director of AbbVie, said in an interview on Monday.

Moreover, Tilton said Treasury left the door open for more changes in tax law, which would have posed additional risk to AbbVie and its shareholders.

JOB SAFE FOR NOW

Jeff Jonas, an analyst with Gabelli & Co, said Gonzalez' job looks safe, as long as he continues to spearhead development of experimental AbbVie treatments for hepatitis C, cancer, endometriosis, multiple sclerosis and other drugs that could be approved before Humira loses ground to generics in the next few years.

"Overall he's done a great job diversifying the company away from Humira," said Jonas, whose company sold its AbbVie shares last year. "Their R&D portfolio is very strong." But Jonas said AbbVie needs to remain on the hunt for other deals to bolster its drug lineup.

In its agreement to buy Shire, AbbVie was likely unable to renegotiate the terms of the merger in the case of extenuating circumstances, such as changes in U.S. tax laws, because British Takeover Panel rules are very stringent and don’t allow for any amendments to a deal structure once it is announced.

"Some shareholders will be disappointed in (Gonzalez') performance," said Marshall Gordon, an analyst with ClearBridge Investments, which holds 1 million shares of AbbVie. "But I think he tried to put a good deal together and unfortunately the government changed the rules midstream in a way that was hard to predict."

Oliver Pursche, co-portfolio manager with Gary Goldberg Financial Services, however, said Gonzalez' survival is anything but assured.

"If AbbVie's stock rebounds and goes up, and Treasury gets Congress to enact legislation that would make inversions unattractive, Gonzalez will get some credit. But the ultimate determinant will be the stock price, and that's a question that won't be answered for six months or longer."

Gonzalez elevated his profile with the ambitious run at Shire, said Raghuram Selvaraju, an analyst with Aegis Capital.

"Gonzalez was considered faceless until he came up with the Shire idea," said Selvaraju. "By being so willing to up the ante, he showed he's a gutsy guy."

AbbVie's final offer was equal to more than 10 times Shire's annual sales, Selvaraju said, far richer than the typical formula of 5 to 7 times annual sales for deals that Gonzalez helped negotiate at Abbott during his 30-year career there, including as head of pharmaceuticals, medical products and hospital products.

Gonzalez, who rarely speaks with media, declined repeated requests for an interview. A company official said no photographs of the CEO were available for distribution, a clear indication he prefers remaining in the shadows despite his highly publicized near-deal with Shire.

(Additional reporting by Anjuli Davies and Caroline Humer; Editing by Bernard Orr)