Duffie suffered a partial dislocation of his shoulder while making a tackle late in their 20-16 defeat by North Queensland at AAMI Park, which was the competition leader's third successive loss.
The 21-year-old scored a superb try to open the scoring for the Storm, out-jumping two Cowboys defenders and then managed to plant the ball as he tumbled to the ground.
Coach Craig Bellamy said Duffie could be sidelined for up to six weeks and revealed Slater was unlikely to make his return from a knee injury after five games out against Parramatta in Sydney next Saturday night.
Dane Nielsen missed the Cowboys clash with a corked bicep while fellow centre Will Chambers remains out indefinitely with a blood disorder.
"Talking to (Slater), it wont be next week by the sounds of it," Bellamy said. "We're hopeful, but we're not going to take a risk.
"He's not overly confident so we'll see what happens. (Duffie) going down isn't helping and I'm not sure when Will (Chambers) will be back.
"It's a strange issue but he won't be back until he's 100 per cent, we wont take any risk with his health."
The Storm lacked the finishing prowess that's a trademark of their game against the Cowboys and the loss of Duffie is set to make their attack even more disjointed.
But Bellamy wasn't panicking, saying their performance against North Queensland was a big improvement on their shock loss to the Raiders during which they conceded seven tries.
He said the side had done enough early in the season to afford a few losses.
"If someone said in January that at round 19 you would be leaders of the competition, you would have grabbed it with both hands," Bellamy said. "We are not playing as well as we were earlier in the season but some of that benefit is holding us in good stead as far as the table goes.
"We've got another seven games to turn this around. You would rather it be happening now with seven weeks to go than one week to go."