Wilson reckons Slew, 18, has made a mistake and will regret the switch.
He told the Daily Star: "The transfer window is a total waste of time and I'm delighted to see the back of it.
"I think it's the wrong move for Jordan and I told him so. He needs to learn his trade and won't do that in the reserves at Blackburn.
"He could have stayed with us and maybe gone in January, by which time he'd have had 20 first-team games with us under his belt.
"But this is what happens, I'm afraid. If the Premier League is dangled in front of people, it is very difficult to turn it down.
"In my opinion, his progression would have been best here, but it is down to the player and agent.
"I didn't know when I arrived what I would have left at this stage, but I am delighted and now I know exactly what we have at our disposal.
"There's no point living in the past, I'm not one to do that. If we were lucky enough to go up this season, then nobody would be happier than me. I wouldn't even say to those who didn't want me here, 'I told you so'."