Now the 20-year-old who has been on U.S. national age group teams since his mid-teens is their starting right back.
And he'll get a chance to face the team that let him slip away, Chivas USA, when Montreal travels to Los Angeles for a game on Wednesday night.
''Right away, he was one of the biggest surprises in terms of names on the list,'' coach Jesse Marsch said this week. ''(Striker) Brian Ching was another, but Zarek was pretty big.
''We got lots of calls from other teams about Zarek looking to make a trade with us, and we pretty much said right away 'We're not trading him. We're taking him.'''
Valentin will be in the starting lineup when the Impact try to win a second game in a row after their uplifting 4-1 victory on Saturday over the Seattle Sounders in the re-opening of the expanded Saputo Stadium. Playing in different conferences, Montreal and Chivas have identical 4-7-3 records.
In Montreal, the Lancaster, Pa., native gets to play with his boyhood friend and teammate Andrew Wenger, whom Montreal picked first overall in the 2012 MLS SuperDraft.
Valentin was taken fourth overall by Chivas in the 2011 SuperDraft out of the University of Akron, where he had won an NCAA championship.
How he was left exposed in the expansion draft likely had much to do with the league's complex salary cap system.
He was listed as a Generation Adidas player, a class of prospects who are signed to relatively high salaries but whose pay is not counted in the team's cap for their first two years in MLS.
However, because he played 25 games and his 2,114 minutes ranked fifth on the team, the league ''graduated'' Valentin out of Generation Adidas after only one season. That left Chivas with a base salary of US$90,000 (and a guaranteed $142,000) on their cap for 2012. They elected to leave Valentin unprotected.
Montreal made him their second choice, just behind Ching, who was unhappy in Montreal and was traded back to the Houston Dynamo for future considerations.
With Chivas, Valentin was part of a defence that struggled last season and had some injuries, which was partly why he got a lot of game experience as a rookie.
''As a rookie playing in the back, you're going to make mistakes and that's what he did,'' said Marsch. ''And I think he's now more mature and steady.
''Having a fresh start in a new environment has been good for him. He's only going to continue to get better and to grow. His major hurdle has just been to eliminate the mistakes. He's got to the point where he's smart on the field, more focused and confident, and all his natural, good qualities start to come up.''
Valentin, a steady fullback and clever passer, said he has no animosity toward the team that let him go and that, other than missing the California beaches, he's glad to be on a promising new team in Montreal.
He was away for the start of the season with the U.S. under-23 squad and then missed some time with a right calf injury suffered April 30 in a 2-0 win over Portland. Since play resumed Saturday after a three-week break, he said he is fully fit.
Valentin is one of four players taken in the expansion draft who should start at Chivas along with left back Jeb Brovsky (picked fifth from Vancouver) and midfielders Justin Mapp (third from Philadelphia) and Collen Warner (sixth from Salt Lake). Defender Josh Gardner (seventh from Columbus) and forward Sanna Nyassi (eighth from Colorado) are also on the roster.
The Impact will be missing a key player as central defender Matteo Ferrari suffered a right thigh injury against Seattle. Veteran Shavar Thomas moves into that spot.
And forward Bernardo Corradi, who was unlikely to start, limped off the practice field on crutches on Monday with an ice pack on his left knee.
The Impact hope to build on the big win against the Sounders in a busy week that sees them return home Saturday to face Houston. Midfielder Patrice Bernier is coming off a three-assist effort that earned MLS player of the week honours. Mapp, Wenger, Felipe Martins and Lamar Neagle all scored goals.
''It reinforces what we've been trying to do,'' captain Davy Arnaud said of the win. ''It was more about making a statement to ourselves: That we understand that we have good players who can make plays and we're tough to play against and we're capable of these kinds of results.''