Stubbs backs new boy Allan
In on a season-long loan, the 24-year-old is out to prove his worth again after struggling to break through at Parkhead.
"He is a very intelligent footballer," Stubbs told the Advertiser. "He likes to make things happen and he's got really good technical ability. He has an eye for creating opportunities for players in front of him and he's very good on the ball.
"I know for a fact he's looking forward to playing more than what he has done at Celtic.
"I know all about him, I know his attributes and I think he can be a really valued addition to the squad."
He becomes Rotherham's seventh summer signing, following goalkeeper Lewis Price, centre-half Kelvin Wilson and midfielders Will Vaulks, Jake Forster-Caskey, Anthony Forde and Jon Taylor.