Martin O'Neill says the hard work will continue after seeing his side round off their pre-season campaign.
Sunderland went down to a 72nd-minute goal from substitute Tom Parkes in the Midlands following the midweek draw at Derby.
"We have plenty to work on," O'Neill said post-match after returning to his former club. "There are a number of areas which need strengthening.
"Today, the second half was played at a better tempo than most of Wednesday night [at Derby].
"It's Leicester's final game of the pre-season too before they kick off and I think the game had more of a competitive edge to it."
Meanwhile O'Neill admits he's missed the likes of Phil Bardsley, who has spent almost the entire pre-season campaign sidelined with an injury.
The Sunderland boss is also hoping Stephane Sessegnon will soon be fit to return after suffering an ankle problem.
"Preparation has been fine but you can always argue the case that the most important thing is getting players onto the field of play," he said.
"We've been unable to do that with Phil, who was absolutely flying at the start and then hurt himself and has missed all of pre-season.
"As well, Sessegnon has come back and picked up an ankle problem, which we hope isn't too bad."