De Rosario was injured during a closed-door scrimmage against the third-division Richmond Kickers last Friday and has not trained at full strength this week. He did not travel with the team to Kansas City.
United scheduled the game against the Kickers because it had the weekend off from league competition. The adductor muscle is in the upper thigh, near the groin.
The club is holding out hope he will be ready for next Saturday’s match against the New York Red Bulls at RFK Stadium.
De Rosario missed the first two matches of the season while serving an MLS suspension. In his absence this time, Coach Ben Olsen might insert Marcos Sanchez into the starting lineup or turn to Marcelo Saragosa, who sat out the previous match with a minor leg injury. Sanchez is back from Panamanian national team duty.
De Rosario joins three other attacking players sidelined with injuries: Nick DeLeon (hamstring) is out for at least a month, Lewis Neal (groin) is out indefinitely and John Thorrington (knee) is ahead of schedule but will not return for probably another month.
Meantime, third-choice goalkeeper Andrew Dykstra has joined Richmond for its season opener Saturday night against the Pittsburgh Riverhounds. He is the fifth United player to go on loan to the Kickers, joining Taylor Kemp, Casey Townsend, Conor Shanosky and Michael Seaton.
Dykstra will train during the week with United and report for matches. Seaton is currently on assignment with Jamaica’s under-17 national team at CONCACAF’s World Cup qualifying tournament in Panama.