A fans' favourite during his time at the West Yorkshire club, he added: "Who comes out of the blocks first is important. As soon as that first whistle goes you have to be at the races.
"The players have to turn up and have no regrets when they are on the bus on the way home."
Lillis wore his heart on his sleeve when he played for the Terriers, scoring 63 goals in 242 games between 1978 and 1985 before going on to play for Manchester City, Derby County, Aston Villa, Scunthorpe United, Stockport County and Macclesfield Town.
He recently left the club after more than four years in charge of the academy - along with four spells as caretaker manager - and is now looking for a new challenge.
The 56-year-old still lives in Huddersfield, and his passion for the club is clear along with his admiration for Wagner.
"He has brought something new to the club," Lillis said.
"He is very meticulous in the sessions and he has the trust and respect of the players."
But it will be the players who need to perform at Wembley if Town are to reach the top flight for the first time in 45 years - Lillis believes victory would be life-changing with financial experts Deloitte estimating promotion to the Premier League is worth at least £170million.
"The supporters will be key too. I know what Town fans are about – all they want is for the players to roll their sleeves up and have a go."