The Everton manager was speaking to ESPN, after his team's successful 1-0 Europa League win over Ruzomberok in Slovakia.
The Dutch side were undone by their inability to score over two legs as Petar Bockaj's 25th minute goal put the Croatian club through 2-0 on aggregate.
"It was a physical game and 90 minutes that the players needed".
"We go through to the next round and I'm really happy".
Having actually taken in the Slovakians' honest efforts at trying to break down their three-man defence, Everton capitalised as the game opened, as Klaassen launched the lively Calvert-Lewin via on objective to port house with a well-placed right-foot shot.
He said: 'This competition has always been important for me.
If Dominic Calvert-Lewin has reason to pinch himself at how his summer has progressed, then Ronald Koeman followed the forward's latest match-winning intervention by delivering a brutal dose of reality.
"I like to be in Europe - it's important for our experience".
The Blues would book a place in the Europa League's group stages if they overcome Hajduk, who remain unbeaten in seven prior visits to England.
"But we know we can, and we need, to improve in our ball possession".