John Requejo Jr., a former U.S. youth international who played in the Mexican leagues for 3 1/2 years, has begun a trial with D.C. United.

The 21-year-old free agent is a left back from Santa Barbara County, Calif., who passed up an offer from UCLA to sign with Tijuana. He played primarily in Copa MX matches for three seasons before going on loan to second-division Dorados last summer, starting five matches across all competitions.

John Requejo Jr. (USSF)

In 2015, Requejo was a member of the U.S. Under-20 World Cup squad in New Zealand with, among others, Paul Arriola, a Tijuana teammate who is now entering his first full season with United. Previously, Requejo played for the under-14, U-15 and U-17 national teams.

D.C. is searching for depth at the outside back positions. The only holdovers from last season are projected starters Taylor Kemp (left) and Nick DeLeon (right).

Also in training camp are right-back candidates Sheanon Williams, an eight-year MLS veteran, and Chris Odoi-Atsem, whose contract option was not renewed by D.C. after an injury-plagued rookie season. United remains in pursuit of a Latin American outside back; the club has declined to identify him publicly.

Midfielder Rob Vincent has also arrived at camp in Clearwater, Fla. He missed all of last season with knee problems that have jeopardized his career. Even if he regains full health, he is a longshot to earn a contract.

Two fourth-round draft picks, VCU midfielder Rafael Andrade Santos and Albany forward Afonso Pinheiro, also face long odds of making the squad.

United continues to pursue an attacking midfielder from Latin America; he, too, has not been publicly identified.

“Nothing has changed,” Coach Ben Olsen said when asked to provide a search update. “There are some targets, but those targets keep moving. Sometimes being patient is a good thing.”

Arriola, Ian Harkes and Russell Canouse reported to camp Tuesday after three weeks at U.S. camp. They did light work in the early session and engaged in regular drills late in the day.

United will conduct an intrasquad scrimmage this weekend before facing Swedish champion Malmo on Feb. 8 in Bradenton, Fla. Three other tuneups are on the agenda: Philadelphia Union on Feb. 17 and second-division Jacksonville Armada on Feb. 21 in the Tampa area, and the second-flight Las Vegas Lights in Nevada on Feb. 24.

United’s season opener is March 3 at Orlando City.