After finishing as runners-up to Paris Saint-Germain in Ligue 1 last season, Monaco are once more expected to challenge in France's top flight despite losing playmaker James Rodriguez to Real Madrid.
However, Jardim - who replaced Claudio Ranieri in the close-season - witnessed his side fall to a 2-1 loss at Stade Louis II on Sunday in the club's season-opener, with a Vincent Aboubakar penalty and Valentin Lavigne's late strike for Lorient sandwiching Radamel Falcao's spot-kick.
Jardim confirmed in his post-match news conference that an addition to his backline is a priority, and felt a sloppy start to the match - during which Aboubakar scored - hindered his team.
"It is a feeling of dissatisifaction," said the 40-year-old. "The first 15-20 minutes were difficult. There were a dozen missed passes and the opponent took advantage on the counter-attack.
"It takes more than technical quality of our passes. We must continue to work on the defensive sector. A player is expected in his position."
Jardim also stated he is looking to improve several areas of his team before the transfer window closes, adding: "Monaco are looking for a central defender, creative players, midfielders and wingers.
"I understand that the fans are worried. We want to win and offer quality football. I'd rather lose in pre-season than during the first day. But we can promise the fans that we will work hard to keep moving forward."