It's likely that when the Union open their seventh Major League Soccer season Sunday at FC Dallas, they will be without two key midfielders.

The ailing Maurice Edu has yet to practice with the team, and Tranquillo Barnetta didn't participate in Tuesday's double session workout, the first since the Union returned on Sunday from preseason camp in Florida.

Edu had sports hernia surgery in October, but Union coach Jim Curtin said that isn't what has kept him out. According to Curtin, Edu is battling a stress reaction in his leg.

"Nothing is broken or anything bad like that, but before he is cleared to physically train with us, he has to have it fully healed," Curtin said.

Barnetta, a member of three World Cup teams for Switzerland and a late-season addition to the Union last year, has tendinitis in his knee, according to Curtin.

"It has slowed him a bunch," the coach said.

Curtin said that Barnetta isn't quite fully fit yet.

"These are two leaders, captain-type players and two guys we miss greatly and two important pieces that may or may not be with us [in] week one. But we have to prepare accordingly," Curtin said.

The Union have great depth in the midfield and are likely to display it during the opener since it would be a surprise if Edu or Barnetta sees action.

Curtin wouldn't name his starters for this week, but he continued to praise the player who has been the biggest surprise in training camp, former Drexel standout Ken Tribbett. He is vying for a starting spot at center back. Tribbett was originally signed to the Union's United Soccer League affiliate, the Bethlehem Steel.

"He has held up against the top guys in our league," Curtin said of Tribbett's play during the preseason. "I am not naive. When [you are] on the road in Dallas [in] week one, it is a lot harder. But, at the same time, he has shown through his body of work that he belongs in the league."