(AP) -- Imagine the team U.S. coach Caleb Porter could have for this month's Olympic qualifying tournament if European clubs cooperated.

Porter filled the last slot on his 20-man preliminary roster Tuesday by adding forward Tony Taylor, who plays for Estoril Praia in Portugal. More changes could come before the March 20 deadline for final rosters, though Porter's choices are limited by some foreign clubs' reluctance to release players.

"Obviously, reading between the lines, there are players of under-23 age that could be in qualifying and, in all honesty, we've already been in conversations with those clubs," Porter said on a conference call. "We've asked for their release and, in several situations, invited the players into camp. At this point, we have not gotten their release so we need to certainly keep our options open until March 20th.

"At the same time, we always knew this was a possibility ... and prepared with alternates."

Clubs are not obligated by FIFA to release players for men's Olympic qualifying, which is limited to players under 23, or even for the games themselves. Senior national team mainstays Jozy Altidore, Timmy Chandler and Danny Williams are eligible for the U-23 team, but all get significant playing time with their European clubs and aren't likely to be released until the Olympics - if at all.

Defender Alfredo Morales was expected to be on the qualifying roster after appearing at the team's February training camp and starting an exhibition game against Mexico. But Hertha Berlin, which is fighting to avoid relegation, apparently has been reluctant to release him.

"We're very confident in the group that we have," Porter said. "We're realistic about the situation, and there are players that have not gotten released. At some point, we need to move on and focus on the guys we can get into camp."

The U.S. starts Olympic qualifying in the North and Central American and Caribbean region against Cuba (March 22), then plays Canada (March 24) and El Salvador (March 26), with all three games at Nashville, Tenn.

The other group includes Mexico, Honduras, Panama, and Trinidad and Tobago, and is based in Carson, Calif. The top two teams in each group advance to the semifinals on March 31 at Kansas City, Kan., and the semifinal winners qualify for the London Olympics.

The preliminary qualifying roster features 10 starters from the team that beat archrival Mexico 2-0 last month, including Ghent midfielder Mix Diskerud and New York Red Bulls forward Juan Agudelo, who scored against El Tri. Eight players have made appearances with the senior national team, including Dallas midfielder Brek Shea, who has played in all 10 games since Jurgen Klinsmann took over, and Philadelphia midfielder Freddy Adu.

Porter also included Molde defender Josh Gatt, who missed last month's training camp because of an ankle injury.

"I like the group that we have from a personnel standpoint," said Porter, who is head coach at Akron and took over as the U-23 coach in October. "We've got a really nice run of momentum going ... we've had good results, good performances, and the players really enjoy the way that we're approaching things. ... They've bought into (the program), and I know they're looking forward to executing it in qualifying."

If the Americans qualify for London, they will be able to add three players over the age of 23. Clint Dempsey and Landon Donovan both have expressed interest in playing, though they also would need their clubs' permission. Dempsey is with Fulham in the English Premier League, while Donovan plays for MLS champion Los Angeles.

The roster:

Goalkeepers: Bill Hamid (D.C.), Sean Johnson (Chicago)

Defenders: Josh Gatt (Molde, Norway), Perry Kitchen (D.C.), Ike Opara (San Jose), Kofi Sarkodie (Houston), Zarek Valentin (Montreal), Jorge Villafana (Chivas USA)

Midfielders: Freddy Adu (Philadelphia), Joe Corona (Tijuana, Mexico), Mix Diskerud (Ghent, Belgium), Jared Jeffrey (Mainz, Germany), Amobi Okugo (Philadelphia), Michael Stephens (Los Angeles)

Forwards: Juan Agudelo (New York), Terrence Boyd (Borussia Dortmund, Germany), Teal Bunbury (Kansas City), Joe Gyau (Hoffenheim, Germany), Brek Shea (Dallas), Tony Taylor (Estoril Praia, Portugal).

SI Apps
We've Got Apps Too
Get expert analysis, unrivaled access, and the award-winning storytelling only SI can provide - from Peter King, Tom Verducci, Lee Jenkins, Seth Davis, and more - delivered straight to you, along with up-to-the-minute news and live scores.