Vancouver coughed up a one-goal lead after a second-half injury felled O'Brien on Saturday and the Whitecaps lost to Seattle 3-2, costing the team its first-ever win against the Sounders.
"When we lost him, the game opened up," Whitecaps head coach Martin Rennie said. "We were disjointed. I don't think any team in MLS can lose three — really, we've lost four — centre backs and be fine."
O'Brien went down in the 62nd minute with a hamstring strain. He said he felt it stretch when he reached for a ball. Not helping him was the field surface, temporary grass laid on top of the artificial turf at CenturyLink Field.
With O'Brien's injury, Vancouver will now miss him, Jay DeMerit, Brad Rusin and Carlyle Mitchell at that position for the foreseeable future.
"I don't think (the field) was great in the sense that it probably contributed to my injury," O'Brien said. "It was unpredictable with the bounces, and it's not ideal."
Goalkeeper Brad Knighton went further in condemning the surface.
"The field was ridiculously terrible," he said. "It's heavy, it's bumpy, the turf's coming up. It's a weird surface to play on, and anytime you put grass on top of turf, you're going to get that."
Neither O'Brien nor Rennie could estimate how long the veteran defender would be out of action.
Greg Klazura entered the match for O'Brien, and he had to play out of position. Normally a wide defender, Klazura looked out of place in the middle. He conceded the penalty kick that tied the game after Vancouver took an early 2-1 lead.
Camilo Sanvezzo gave the Whitecaps that lead with two headers, assisted by Russell Teibert. Both goals came in a 16-minute span in the first half.
"There are lots of things for us to build on from this game," Rennie said. "Camilo also had a fantastic free kick that was rebounded that we really had a good chance to make that 3-1. At that point, it would have been hard for them to get back."
Sanvezzo added: "It's positive that I scored, but I think the team played well. … I think everybody had a good game, and we just need to pick up from here."
However, Vancouver failed to capitalize on Sanvezzo's free-kick opportunity, and in the second half, Seattle scored two goals of its own in a 10-minute burst to tie and win the game. Both goals came after O'Brien left with his injury.
"When you lose someone like [O'Brien] and you have to play your left back at right-centre back … that can happen," Rennie said.
Sounders FC expanded its stadium capacity for the visiting Whitecaps, recognizing the game's appeal because of the regional rivalry. The official attendance was 53,679.
Seattle has never lost in front of a home crowd larger than 50,000.
"It was a great contingent of fans from Vancouver as well that followed their team," Sounders FC head coach Sigi Schmid said. "It had a tremendous atmosphere."
As a result of the win, Sounders FC (6-4-3, fourth place in Western Conference) also took the lead in the Cascadia Cup. They sit in first place, above the Portland Timbers in second and Vancouver in third, in the fan-created tournament.
The Whitecaps (4-5-4, seventh place) return home for two consecutive matches, on June 15 and 19 against the New England Revolution and Chivas USA, respectively.
"Our home form has been very good, so we need to perform really well in those games," Rennie said. "It would be nice to get six points, and I think we're definitely capable of that."
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