The 32-year-old England midfielder sustained an ankle ligament injury in training earlier this week and underwent an operation on Thursday morning, ruling him out of action for between 10 to 12 weeks.
"Michael Carrick was injured in the last day or the day before," Louis van Gaal, speaking at his first press conference as United's new manager, told reporters.
"So that is, in my opinion, a big blow because he is an experienced player."
Asked how long it could take for Carrick to recover, the former Netherlands coach said: "Much too long for me."
The Premier League side travel to the United States for a pre-season tour on Friday with the midfielder staying behind to work on his recovery.
United host Swansea City in their first Premier League fixture of the new season on Aug. 16.
(Reporting By Sam Holden; Editing by Rex Gowar)
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