Executive defections have been a common sight at Yahoo! (Nasdaq: YHOO), but some of the departures will hurt more than others.

BoomTown's Kara Swisher reports that Andrew Siegel will leave the company. As the head of corporate development, Siegel orchestrated Yahoo!'s acquisitions.

Sure, he lost out on a few of the big fish. Rumors of Foursquare and Groupon buyouts failed to materialize. He's also parted with some of his own company's holdings. Siegel was there when Yahoo! decided to sell HotJobs to Monster Worldwide (NYSE: MWW) earlier this year.

However, in his brief tenure -- he came over from General Electric (NYSE: GE) just last year -- he sealed the acquisitions of Dapper, Associated Content, and other bite-sized deals.

Yahoo! never broke the bank, and it was able to resist the urge to unload its valuable Asian investments.

Despite the revolving door of executives, there's no reason to doubt that Yahoo! can find a capable replacement. My concern here is that Siegel's departure comes at the worst possible time. Google (Nasdaq: GOOG) is on the verge of completing its largest acquisition ever. Outsourcing its search operations to Microsoft's (Nasdaq: MSFT) Bing all but forces Yahoo! to acquire more user-enticing websites to offset its slowdown in organic growth.

The pace of consolidation among dot-com properties is brisk, and Yahoo! can't afford to miss out on the frenzy.

Cynics may argue that Siegel is leaving so quickly because he sees the writing on the wall. The inevitable pairing of Yahoo! with AOL (NYSE: AOL) as the Internet's two traffic-rich, monetization-poor giants will likely lead to further executive defections. Siegel may as well get out ahead of the pack.

However, I would argue that Yahoo! would sooner cash out its Asian investments and use the proceeds to go on the mother of all shopping sprees, if that's what it needs to avoid becoming second banana in a merger with AOL.

Now more than ever, Yahoo! needs to make a defining acquisition. Too bad it'll first have to waste time filling a newly empty chair.

What do you think Yahoo! should buy next? Share your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz wonders if Yahoo! CEO Carol Bartz regrets her decision to lead Yahoo!. He does not own shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy, and it's got mail.