Merck & Co has said it will proceed with its planned $8.4 billion acquisition of Cubist Pharmaceuticals in the first quarter of 2015 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, Cubicin.

The purchase would give Merck, the US market's second largest drugmaker, entry into the market for drugs that fight so-called superbugs.

A day before the Merck statement, Judge Gregory Sleet in Delaware had invalidated four Cubicin patents and ruled that Hospira can launch a generic version of Cubicin as soon as 2016 - two years sooner than Wall Street expected.

Merck noted that Sleet's decision is subject to appeal.

"The combined strength of both companies will provide both incremental and long-term value, and Merck expects the transaction to add more than $1 billion of revenue to its 2015 base, with strong growth potential thereafter," the US drugmaker said in a statement.

If Sleet's ruling stands, Leerink Partners analyst Seamus Fernandez told the news agency Reuters he expects several less expensive generic forms of Cubicin to be introduced in the US by late 2016.

Fernandez said lost sales from the Cubicin patent expiry suggest the price Merck may be paying is $2-3 billion too much. This is a "very tough start to a relatively sound strategic deal," he said.