Celtic youth development chief Chris McCart is an admirer of Slane, who has missed much of the season through injury, after working with the teenager during his stint in the same job at Fir Park.
Slane made his debut last season but rose to prominence during Motherwell's Europa League campaign under Gannon.
He made his return from two calf operations during the Boxing Day defeat by St Johnstone, coming off the bench for his second SPL appearance of the season.
But reports claim he is about to commit his future beyond the summer to Celtic.
"I've heard rumours to that effect but Paul has been ill the last couple of days, genuinely," Brown said.
"Hopefully he will be back today and we can have a chat with him.
"We haven't had any contact from Celtic Football Club and I do think that they would have the courtesy to phone and say to someone in the club that they were going to put one of our players on a pre-contract.
"So I would think it's most unlike Celtic. I would think that's the rumour factory in operation."