With one of the league's youngest backlines in 2018, the Philadelphia Union might get even younger in 2019.

On Wednesday, the club dealt fullback Keegan Rosenberry to the Colorado Rapids for $300,000 in allocation money, that could rise to $400,000 if certain thresholds are met. The 25-year-old played 80 MLS regular season games for the Union over the last three seasons.

Union sporting director Ernst Tanner explained his reasoning behind the move.

"It was a tough decision," Tanner said on a media call after the deal was announced. "There were some reasons: it was his last contract year, we got a good number and we have good players in addition, so we went for that deal. We wish Keegan the best for his future."

The move opens minutes on the backline for 21-year-old Cameroonian fullback Olivier Mbaizo.

"It doesn't have anything to do with the style of play," Tanner said. "The decision was a logic decision, we have the position covered with a very good prospect Olivier Mbaizo who really showed us that he can play the position well. In addition, we are in negotiations with Ray Gaddis, who could do better on the right than the left."

With Homegrown 19-year-old left back Matthew Real making his professional debut last season, Mbazio could be the oldest member of the backline at times next season. With young center backs Auston Trusty (20) and Mark McKenzie (19) set to be the club's first-choice pairing, the backline will be youthful, something Tanner doesn't mind.

"We did play with young guys in the back last season, I'm very confident of the players we have there," Tanner said. "For me, age doesn't matter. Performance matters. We will for sure have one experienced guy who will join us in addition, but I don't have a problem with a young prospect team as long as the performance is good enough."

But there will be additions, specifically another left back.

"We need a left back, that's without any question," Tanner said. "We need to take into consideration that Matthew Real will not be available because of the U-20 World Cup. It's absolutely necessary."