It's possible to overcome a deficit after the first game, but not easy:
Just last season, the San Jose Earthquakes lost the first game to the visiting New York Red Bulls, 1-0. Then San Jose went on the road to win 3-1 in the second game and claimed the series with a 3-2 goal advantage.
In 2009, New England beat visiting Chicago, 2-1, in the first game of the Eastern Conference semifinal only to lose, 2-0, at Chicago and fall, 3-2, in the aggregate total.
The most classic comeback came in 2003 when Los Angeles beat San Jose in the first Western Conference semifinal game, 2-0, and led by 2-0 in the second. San Jose then scored the next five goals in a 5-2 overtime win to take the aggregate series, 5-4.
Veteran Union midfielder Brian Carroll has been on teams that qualified for the playoffs every year since he joined MLS in 2003 and his teams have never overcome a deficit after the first game of an aggregate-goal series.
"There is always a first time for everything," Carroll said after a light workout Monday at PPL Park.
Carroll said the Union fully expect their season to continue after Thursday. The winner will face the Kansas City-Colorado winner in Sunday's Eastern Conference final. Kansas City earned a 2-0 win over Colorado in the first leg of their series and will host Colorado in the second leg on Wednesday.
"I don't think anybody in the locker room thinks this is an impossible feat," Carroll said. "The season isn't over and we have to go down there and prove to everyone we can win a road game in Houston."
In their brief history, the Union are 2-0 at Houston's Robertson Stadium.
Close calls. After looking at replays, the Union, while not dwelling on it, felt that they were on the short end of a few calls and no-calls. The team felt that Calen Carr was offside on Houston's second goal in the 30th minute.
"From all angles we look at Carr, he seems to be offside," Union assistant coach John Hackworth said Monday.
The Union felt another controversial no-call was when Houston's Brad Davis wasn't called for a hand ball inside the box on a Sebastien Le Toux free kick in the 54th minute.
"After looking at game film, it is obvious where a couple of incidents were that we were really unfortunate not to get a different decision," Hackworth said.
One thing that was problematic on both plays was that there was only an angle of Davis from the back, so even though it seems like it was a hand ball, it's difficult to say for sure. As for the so-called offside call, that is best described as being up for debate.
The Union admitted that if they had cashed in on their scoring chances, the calls or no-calls wouldn't have been a factor.
Contact staff writer Marc Narducci at 856-779-3225, firstname.lastname@example.org, or @sjnard on Twitter.