The 21-year-old has become the club's tenth new recruit in the current window, with the Sheffielder having progressed through the youth ranks at Bramall Lane.
Slater became the Blades' youngest-ever goalscorer when netting on his senior debut against Grimsby in the EFL Trophy in 2016 and has spent the past two seasons out on loan in League Two - at Carlisle United and Scunthorpe United respectively.
On signing Slater, McCann commented: "Regan’s someone we’ve been monitoring for quite a while.
“He’s a really good player, really sharp in his movements, really energetic, calm on the ball and has really good legs in terms of running away from people.
“He’s had two very good loans at Scunthorpe and Carlisle, where he racked up over 40 appearances. He’s ready for the next step.
“He was with Sheffield United’s first team all pre-season, and played in their pre-season games.
"He’s thought of highly there so we’re pleased to get him. He’s a good kid, got a good attitude and is a really good character.
“Our midfielders have done well but to bring another one in helps us and Regan will come in and push them all the way.”