Jackett's side have struggled in the league in recent weeks and were beaten in their last home game 2-0 by Wolverhampton Wanderers.
The manager has said that his squad are determined to improve their performances, starting with the FA Cup.
"The chance to play at Wembley is a fantastic incentive for the players," the Evening Standard quotes Jackett as saying. "They've been different types of games throughout the run and we've negotiated all of them, from a Premier League team to a non-league team.
"We are really focused on this game and there is a good buzz at the training ground. We need to improve from how we played against Wolves and we will improve on Sunday."
Millwall are currently 15th in the Championship.