Sydney FC will go into their must-win AFC Champions League game with a different looking defence and adamant they won't be distracted by next Sunday's A-League semifinal against Melbourne Victory.
Left-back Michael Zullo will start in Tuesday's away clash with group leader Ulsan Hyundai after missing four matches with a hamstring injury.
Coach Steve Corica opted to leave his central defensive linchpin Alex Wilkinson back home because of a tight Achilles.
Tratt has made just three A-league starts for Sydney and has yet to appear in the Champions League, but Corica said both he and fellow youngster Calver were playing well.
"I had them in my youth team here at Sydney and they played together at centre back a long time ago now, but they have got a good combination going there," Corica said.
Tratt was signed on a short-term injury contract last November but Corica wants him to stay on for next season.
Sydney enter the penultimate round of Champions League group games last of the four teams, but just two points off second. Ulsan lead on eight points.
Melbourne Victory left 10 players back home including all five imports, for their Champions League clash in South Korea with with Daegu FC on Wednesday.
"I wouldn't say [that's] an advantage [for them over us] but we play to stay in the Champions League," said Corica, who pointed out Victory had no realistic chance of advancing beyond their group.
Corica was adamant his players focus on Tuesday's game wasn't being diluted by thoughts of next Sunday's match.
"The boys that are here, I've already spoken to them about one game at a time, obviously only focus on this game," Corica said.
"We need to win this game first and foremost and then we take the semifinal after that."
Sydney dominated their home game against Ulsan but were held to a goalless draw by a side that was happy to sit back.
He doubted whether they would be much more attack-minded at home.
Corica noted Ulsan lost their latest domestic game after conceding a couple of goals from counter-attacks and felt Sydney could exploit that as well.