Captain Dylan Carreiro (Dundee) and fellow midfielder Fraser Aird (Rangers) were not released by their clubs.
"(I'm a) bit disappointed to lose some key characters with clubs not releasing them but it will provide opportunities for talented players to step up and prove they can apply themselves at this level," Canadian coach Rob Gale said in a statement Monday.
Teams do not have to release their players for FIFA youth competitions.
"Sad to not be involved in the qualifiers, but I am more than positive the boys will do the job and get us to the World Cup in June," tweeted Carreiro.
Canada opens Saturday against Haiti in Group B play with games to follow against Mexico, El Salvador, Cuba and Honduras. All of Canada's contests are in Montego Bay.
Group A features host Jamaica, Trinidad and Tobago, Aruba, Panama, the U.S. and Guatemala.
The group winners move to the tournament final and advance to the FIFA U-20 World Cup in New Zealand. The second- and third-place teams will cross over to play the second- and third-place countries from the other group to decide the final two World Cup spots from North and Central America and the Caribbean.
Canada has not taken part at the FIFA U-20 championship since it hosted the event in 2007, when it failed to score a goal or pick up a point.
The Canadian roster includes six players from the Vancouver Whitecaps organization: goalkeeper Marco Carducci, defenders Samuel Adekugbe, Jackson Farmer and Jordan Haynes and midfielders Marco Bustos and Kianz Froese.
Toronto FC forward Jordan Hamilton will help lead an attack that also features Calum Fergusson of Scotland's Inverness Caldeonian Thistle and Cyle Larin, a collegiate star in his first two years at UConn.
Canada Soccer listed Larin as unattached, leading to speculation that the star forward, who has already been capped by the Canadian senior team, is leaving UConn.
There was no immediate word on Larin's future from UConn but it appears he may be headed to the MLS SuperDraft on a Generation Adidas contract, given to top underclassmen who leave early.
Mexico is the two-time defending champion, having won the CONCACAF under-20 crown at home in Puebla in 2013 and in Guatemala City in 2011.
Canada U-20 team
Goalkeepers: Marco Carducci, Vancouver Whitecaps; Nolan Wirth, Oregon State University.
Defenders: Samuel Adekugbe, Vancouver Whitecaps; Alexander Comsia, RC Strasbourg (France); Luca Gasparotto, Airdrieonians on loan from Rangers (Scotland); Jackson Farmer, Vancouver Whitecaps; Jordan Haynes, Vancouver Whitecaps; Brandon John, Erzgebirge Aue (Germany); Rares (Chris) Serban, University of British Columbia.
Midfielders: Manuel Aparicio, Toronto FC, Louis Beland-Goyette, Montreal Impact; Hanson Boakai, FC Edmonton; Marco Bustos, Vancouver Whitecaps; Kianz Froese, Vancouver Whitecaps; Jeremy Gagnon-Lapare, Montreal Impact; Chris Nanco, Syracuse University; Michael Petrasso, Queens Park Rangers (England).
Forwards: Calum Fergusson, Inverness Caldeonian Thistle (Scotland); Jordan Hamilton, Toronto FC; Cyle Larin,| unattached.
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