Wilder said the two loan signings will be involved in the FA Cup fifth-round tie at Reading. Centre-back Retsos, on loan from Bayer Leverkusen, has been an unused substitute in the last two matches, whereas centre-forward Zivkovic, who has come from Changchun Yatai, is yet to make a matchday squad.
Sheffield United have an option to make both loans permanent at the end of the season.
“I'm not going to take a chance because I need to see them in a game, they get into the team through an opportunity, they have to earn the right,” he said. “That doesn’t say we won’t take up their options even if some don’t play any minutes at all. We'll trust our judgement in those cases.
“They're all desperate to play but I don’t want to be sat here (after an FA Cup exit) and say, 'That didn’t work,' because we tried something out.
“We have to pick the right side and the best side for the game to get a result.
“I know I won’t make six or seven changes but looking at injuries and individual games, we have to keep an eye on the schedule too.
“Someone might suit one game but not the others. There is planning, it is difficult because everyone wants to play.”
Meanwhile, Wilder says progress is being made in contract talks with John Lundstram, Enda Stevens and John Egan.
“Discussions between their agents and the club are ongoing but it's not always a quick process to get over the line,” he said. “I respect players want the best deals, and agents do too, and I don’t have any feeling that anyone wants to play their football elsewhere. If we did, we would act on that.
“The reason we’ve offered deals is that we want to build on what’s been a tremendous season so far. Offers will go out and hopefully players will commit as John (Fleck) has done.”