FRISCO, Texas - Keegan Rosenberry will long remember his professional soccer debut.

He'd probably like to forget it.

The Union defender and No. 3 overall choice in the 2016 MLS draft was helpless against the speed of FC Dallas' attack in his team's season-opening 2-0 loss Sunday at Toyota Stadium, finding himself a step behind - often more than one - throughout the afternoon.

Not that he was alone. The Union defense, which also included newcomers Anderson Conceicao and Ken Tribbett, was outrun and thoroughly overmatched by swift Dallas attackers Fabian Castillo, Michael Barrios, and Mauro Diaz as the visitors allowed 18 Dallas shots, 10 on goal.

Diaz wound up assisting on both goals, hitting Castillo with a through ball behind the defense in the 22nd minute, and stripping the ball from Union defender Fabinho before feeding Max Urruti for an open shot in the 79th.

"It lived up to everything I imagined it would be," Rosenberry said of his first MLS match. "It was a tough opponent for us in game one as a team and, obviously, individually, with a couple of tough matchups. But there were some things to build off today, and that's what we want to focus on.

"Defensively, it was a lot to handle - a couple of speedy wingers and tricky guys. But overall, I'm relatively pleased, I guess. I'm kind of a microcosm of the team. I have things I can look back on and say I did well, but there are definitely things I've got to work on."

Still, the Union were able to keep the game from getting out of hand thanks to a stellar performance by young goalkeeper Andre Blake. The 25-year-old Jamaican made eight saves and broke up several more Dallas scoring chances by ranging far out of the net to cover for his beleaguered defenders.

Blake drew raves from his grateful teammates, who knew he was the only thing standing between them and a blowout loss. Coach Jim Curtin, noting Blake was kept far too busy, said, "He bailed us out a lot. . . . But we hung him out to dry far too much, especially at the end of the game."

That was not only the fault of the defense. The Union failed to hold possession of the ball during the first half and couldn't manage to put pressure on Dallas keeper Jesse Gonzalez until the late going.

"We were a bit tentative," midfielder Chris Pontius said. "The best defense for us would have been more possession, making them chase us more. We just weren't able to get on the ball enough.

"But it was the first game: We got the tentativeness out of our system. We're still positive."