Dec 9 (Reuters) - Merck & Co on Tuesday said it will proceed with its planned $8.4 billion purchase of Cubist Pharmaceuticals and still expects the deal to boost its long-term earnings, despite a court ruling that could speed the arrival of generic forms of Cubist's top-selling product.

Merck late on Friday had announced plans to buy Cubist, whose flagship product is the widely used antibiotic Cubicin for skin infections. The deal would give the No. 2 U.S. drugmaker entry into the market for drugs that fight so-called superbugs.

But U.S. District Judge Gregory Sleet in Delaware on Monday invalidated four Cubicin patents and ruled that Hospira Inc can launch a generic version of Cubicin as soon as 2016, two years sooner than Wall Street expectations.

Merck, in a release on Tuesday, said it still expects to complete its purchase of Cubist in the first quarter of 2015, and noted that Sleet's decision "is subject to appeal."

(Reporting by Ransdell Pierson)