The central defender was at the heart of a quality rearguard action that helped the Owls to Saturday’s 1-0 win at Hull City that made sure they could not suffer the embarrassment of relegation.
That had been a lingering concern for the South Yorkshire club who had been fighting for a place in the Premier League over the previous two campaigns.
Injuries, of course, have blighted them, but Lees believes the hard work for 2018-19 must start now in their remaining three Championship fixtures.
“I think it is important we do start now,” said the former Leeds United centre-back.
“There’s a lot of things that need to change as obviously this season has not been good enough.
“When you look at performances now, even when we’re getting players back we should be a team that are dominating games and I thought we got dominated (at Hull) to be honest.
“It’s always good to win and we have not enjoyed the last few games – losing twice and getting battered at QPR – so everyone goes away happy.
“At times we showed how good we can be when we had the ball, especially in the first half.
“But we rode our luck at other times and showed what we have to work on for next season. It shows we are still a bit off. Now until the end of the season is a chance for everyone to show what they can do and put themselves in contention for next year.”
With a rare midweek break Wednesday can enjoy some rest ahead of Saturday’s home game against struggling Reading.
Lees, 27, said: “It’s nice actually to have a week between the games as it’s been a hectic schedule these last few weeks.
“There’s players that are coming back from injury that have played a lot of football in a short space of time so it gives them a good chance to get ready for that next game now.”
Jordan Rhodes will hope to start again against Reading after he came in from the cold to score at Hull, his first goal since Boxing Day.
Lees said: “Obviously he’s not had much game time lately so for him to get a start and the goal will be great for him. We didn’t have too many chances so luckily he took that one very well.”