McDonald has reportedly been acquired in exchange for a third-round draft pick in 2014 and a conditional 2015 pick. Rich Balchan, as a result of McDonald's joining the club, will be placed on the disabled list, as he is projected to miss the rest of the season. A player can only be placed on the disabled list once a replacement has been signed.
McDonald, 27, has played only 12 matches for struggling D.C. United this season; having appeared in all but one match for the club last season, his relegation to the bench is an interesting one.
"...We felt it was important to add a talented experienced center back and we got that with Brandon."RSL GM Garth Lagerwey on McDonald
The announcement comes a day after rumblings that Chris Schuler could return within a month's time, but with center back Kwame Watson-Siriboe out for the season, additional backup was perhaps necessary. Schuler has been somewhat injury-prone through his young career at the club thus far, having suffered from stress response issues in his feet.
It remains unclear whether Real Salt Lake will be paying McDonald's full salary under the cap, or whether D.C. United will offset that; a similar deal was struck when Justin Braun was brought to the club during the 2012 mid-season.
McDonald, ostensibly a veteran, has played 137 matches since coming into the league with L.A. Galaxy; he also played with San Jose Earthquakes from 2009 to 2011 before being dealt to D.C. United.
Interestingly, McDonald and Robbie Findley, both from Arizona, played as youth for Sereno Golden Eagles.
Update: McDonald will wear #44 for Real Salt Lake.
Update 2: Steven Goff of the Washington Post reports that a portion of McDonald's salary remains on DC United's books.