The Serie A side have already recruited Xherdan Shakiri, Lukas Podolski and Marcelo Brozovic this month, but Mancini is eager to do more business.
Juan Jesus, Jonathan and Marco Andreolli all on the sidelines, so adding more defenders to his squad is the Inter coach's top priority.
"At the moment we're working on other areas but it's not easy to find players capable of changing things for you in January," he said.
"Do we need a striker? We need other things. You build a team from the back.
"At the moment we only have one fit centre-back. We certainly need someone in defence and probably in midfield too.
"We're going to be playing three games a week soon and we need as well-rounded a squad as possible."
Inter's defeat against Torino last weekend left them 10th in the table, but Mancini is confident they are making progress.
"It was a bad result against Torino but we must aim to keep improving every day and hope to get a bit more luck while playing well." the former Italy striker said.
"My lads are putting everything into it, both on the training pitch and in matches. Regardless of the result we've made progress on an individual level and as a team.
"Of course my hope is that we continue to improve. We need to score goals and to do that we have to show a bit more flair in the final third and be more clinical in front of goal."
Sassuolo host Inter this weekend and a victory for the hosts would see them climb above Mancini's side.
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