Deila told Si Ferry's Open Goal: "Did I get in touch with City? Yes. I said you have to take him. They said, 'okay, tell me the best European game he has played and we will watch it'.
"I couldn't tell them what game that was because he hadn't had any real good European games. He was really dominant in Scotland but he got sent off against Inter away.
"That was a big test. I think if he had played fantastically in that game, he would have been bought.
"I think the big clubs think Scottish football is too low level and they don't want to take a chance on a player for that kind of money. In Europe, then you can show how good you are. I think van Dijk was good, but he didn't have that one really, really good game in Europe."