The injury, suffered in the 74th minute of Saturday’s home opener against Real Salt Lake, is the same setback that Dwayne De Rosario suffered last fall. United’s captain returned for the last playoff game after a two-month layoff.
“It’s disappointing — it’s a big blow,” Coach Ben Olsen said.
Thorrington, who is 33 and in his first season with United, has a history of injuries but most involved muscular problems.
Marcelo Saragosa is the leading candidate to replace Thorrington in central midfield against the New York Red Bulls this weekend — should Olsen maintain a similar formation.
De Rosario is eligible to play after serving a two-game suspension and expected to line up as a withdrawn forward.