Fellow striker Niasse, who has scored six goals in nine games across all competitions, is serving a two-match suspension after being found guilty of simulation during Everton's draw with Crystal Palace last weekend.
"He (Sandro) has been working harder. We need every single player to contribute, to run around, to work their socks off and that's a prerequisite - we can't carry anybody," Unsworth told the club's website. (www.evertonfc.com)
"There's always an opportunity for players when somebody gets suspended or is injured. That's why you have so many players in your squad.
"Oumar's been doing brilliantly for us and will unfortunately miss the game, which I'm really disappointed about because he's been flying for us. But it gives somebody else an opportunity."
Niasse, 27, will miss Everton's league games against Southampton and their home match against West Ham United on Wednesday.
Everton, who are 16th in the table, host promoted Huddersfield Town following West Ham's visit and Unsworth has urged his team to maintain clean sheets, collect points and climb up the table over the next week.
"That's the positive way to look at it and the way I want to look at it. It's an opportunity to get some points on the board and it could be a great week," Unsworth added.
"But we need to start getting some clean sheets. It's something we've been speaking about and working on on the training ground - the clean sheet mentality and everybody contributing and doing their job when we haven't got the ball."
(Reporting by Aditi Prakash in Bengaluru)
