A strong Hoops side were behind for most of the pre-season friendly at St Mirren Park with midfielder Marek Hanousek helping himself to a hat-trick for the Czech outfit.
Leigh Griffiths, Gary Mackay-Steven and van Dijk scored for Celtic.
And Deila has warned his players they need to step up against Icelandic champions Stjarnan in the Champions League second qualifying round on July 15.
When asked what watching Stjarnan boss Runar Pall Sigmundsson would have made of the game, the Celtic boss said: “I think he saw some possibilities there.
“We can’t play like this or we will get big problems whoever we play, if it’s against Stjarnan or St Johnstone or Barcelona.
“It was a good test for us. They are a good team.
“I don’t think we had a good performance but it’s early. We have been back four days with 60 per cent of the team.”
Deila started with Dedryck Boyata and Charlie Mulgrew in central defence, while new right-back Saidy Janko was on the bench.
The Norweigan said: “Defensively we have good players, it’s not about the quality, we just have to be sharper. We were always half a second too late.
“It’s frustrating to watch but you just have to swallow and look forward to the next training session and match.”
The Scottish champions are next in action in a friendly against David Moyes’ Real Sociedad on Friday at St Mirren Park.