Duff will miss the rest of the A-League season after rupturing a tendon in his calf and damaging ankle ligaments away to Wellington Phoenix last Sunday.
A week later City were held to a 0-0 draw at home to league leaders Perth Glory at AAMI Park, with the hosts missing the energy and experience of the two-time Premier League winner.
"We're looking," van 't Schip said when asked about short-term reinforcements.
"It's not easy because you have to get a quality player in. It has to be a player out of contract. We're busy with it. There are a few things we have to look at. That's something we're dealing with as much as we can, to get someone in who can replace Damien."
City had four shots on target to Glory's none in a forgettable contest, a game van 't Schip felt his team should have won.
"They were more happy with the point than we were.
"The second half I think we were the better team as well. To be honest I thought it was a red card (for a tackle) on Josh," he added, referring to the yellow shown to Dennis Kramar for bringing down Josh Kennedy in the second half.
City have taken a point off the competition's front-runners in each of their last two meetings now, prompting suggestions Glory perhaps aren't worthy of their status as A-League leaders.
"If you see the games against us, maybe not, but that's just a very small percentage of the games they've playing," van 't Schip said.
"They're very effective. You can see why they're on top."
Next up for City is a home match against another of the title contenders, third-placed Adelaide United.
"Next week is going to be a tough game," van 't Schip said.
"Adelaide is a team playing the best football in the league. It's a great opportunity to show we're growing as a team."