The team lost their penultimate pre-season game against the Phoenix Suns 88-76.
The point guard told the team's official website: "We have to have more force offensively and defensively. We kind of got going late. They just got out to an early lead and it was hard to battle back.
Head coach Scott Brooks added: "Every minute is a minute to get better. We have to play our minutes hard.
"We have to be able to defend with toughness, we have to be able to box out and we have to be able to rebound."
Jackson finished the game with 18 points, four assists, three rebounds and two steals.