The Buffalo Sabres shipping star left wing Thomas Vanek to the New York Islanders for winger Matt Moulson and draft picks caught many NHL observers off-guard.

It was the timing of the deal which was so surprising. Vanek was widely expected to be moved at some point this season. The rebuilding Sabres were off to a horrible start and Vanek (eligible for unrestricted free agency next summer) wasn’t expected to re-sign.

Since the implementation of the salary cap in 2005, a trade of such an expensive ($6.4 million in real dollars, $7.142 in cap hit) talent rarely occurs early in the the season. One must go back to November 2005 – when the Boston Bruins dealt Joe Thornton to the San Jose Sharks – to find a similar early-season trade.

The trade of Thomas Vanek (l) to the Isles for Matt Moulson was the first major early season trade under this CBA.

The trade of Thomas Vanek (l) to the Isles for Matt Moulson was the first major early season trade under this CBA.

The Vanek trade, however, was the first to occur in the season’s opening month.

Under a salary cap, it’s very difficult to move a high-salaried star like Vanek. To make the deal happen required not just a general manager desperate to sell (in this case, Buffalo’s Darcy Regier) and a GM with sufficient cap space willing to gamble (the Islanders Garth Snow), but also for the selling GM to absorb part of the salary of the player being shopped to make it work.

Thanks to the new collective bargaining agreement, teams are allowed to retain up to 50 percent of a player’s salary to facilitate a trade.

Regier didn’t have to retain that much of Vanek’s cap hit, but it took absorbing a sizeable chunk (20 percent, or $1.4 million) to entice Snow to make the deal.

It’s unlikely, however, the ability to retain salary will spark a significant increase of early-season trades involving expensive stars.

One reason is most teams have used up the bulk of their payrolls when the season opens in October. That significantly reduces the number of potential trade partners for a desperate GM looking to sell early in the season, especially when shopping players carrying very expensive contracts.

A factor in the Vanek trade was his contract expires at the end of this season. A GM looking to sell is more willing to retain salary on an expiring contract than a multi-year deal.

Not every GM will be desperate enough to retain more than what Regier did with Vanek’s salary. He only agreed on 20 percent. As desperate as Regier was to move Vanek, even he wouldn’t agree to take on half.

The player’s free agent status is also a factor. Snow, in this case, is gambling Vanek will enjoy playing alongside superstar John Tavares and agree to re-sign rather than test next summer’s free agent market.

Most NHL GMs, however, don’t like to part with assets on such a gamble, except near the trade deadline,when most of the player’s remaining salary has already been paid out. Even then, it depends upon where teams are positioned in the standings, and if their GMs feel it’s worth the gamble for a shot at a championship.

If the salary cap steadily rises over the course of this CBA, as it did throughout the previous one, it can be argued such deals will become more commonplace.

It would be foolhardy to entirely dismiss that theory, especially if more teams have available cap space at their disposal, or a willingness to absorb a portion of a player’s salary in order to move him.

The circumstances surrounding the Vanek trade, however, are rare. That’s not to say we’ll never see another early-season trade involving an NHL star over the remainder of this CBA. Just don’t expect it to become a regular occurrence.