NEW YORK — An economics professor at the Massachusetts Institute of Technology testified Monday that there will be “significant price erosion” in the drug Plavix, even if a federal judge grants a request to stop a Canadian company from marketing a less expensive, generic version of the blood-thinner.

Testifying at a hearing in federal court in Manhattan, Jerry Hausman, who has taught at MIT since 1973, said that drug makers Bristol-Myers Squibb Co. (BMY) and Sanofi-Aventis (SNY) may be forced to offer rebates to keep name-brand Plavix from being assigned to a higher-pricing tier for co-pays at third-party payment providers, which administer drug plans for health insurers. Drug makers’ profit margins would decline as a result.

It’s also likely that the third-party providers are stockpiling supplies of the generic, so they could have as much as a year’s supply of the drug on hand by the time an injunction is issued, said Mr. Hausman, testifying as an expert witness for Bristol-Myers and Sanofi-Aventis. As a result, Mr. Hausman said, the third-party providers won’t feel immediate pressure to return to a more favorable price for the drug makers and may insist that they keep a rebate in place, even though there’s no generic competition.

Bristol-Myers and Sanofi have asked U.S. District Judge Sidney H. Stein in Manhattan to issue a preliminary injunction to stop Apotex Inc. from selling a generic version of the drug.

Apotex launched a generic version of Plavix last week, renewing a 4-year-old patent fight over the drug. The companies had reached a deal earlier this year under which Apotex would delay launching a generic version of Plavix until 2011, when Sanofi’s key U.S. patent expires.

However, the proposed settlement was rejected by state antitrust regulators. Last month, the U.S. Department of Justice launched a criminal investigation into the proposed settlement.