The 18-year-old joined the Scottish Premiership leaders on loan from Manchester City.
The 18-year-old - who played alongside the defender during his days at Championship side Fulham - has joined the Hoops from Manchester City on an 18-month loan (via The Scottish Sun), and Grimmer believes that the move to Parkhead will give the prospect the opportunity to showcase the incredible amount of talent he possesses.
"Every day in training he would produce something that would make you stand back and admire his talents - unless you were on the receiving end," Grimmer said, as reported by The Scottish Sun.
"It didn't take him long to get his move to City and obviously first-team opportunities there are hard to come by.
"He will excite when he's on the ball and, if he gets game time at Celtic, folk will see the best of him."
With Ronny Deila's men currently enjoying a six point lead over Aberdeen at the top of the table - which could be extended to nine if they beat the Dons on Wednesday - and having an abundance of exciting attacking-minded players at their disposal, the switch to Celtic is arguably the perfect move for Roberts at this point in his career.
Their commanding position in the table, as well as their strength in depth may allow for Roberts to play his natural game without the pressure of worrying what the consequences may be if he loses the ball high up the pitch.
And having that freedom - as well as winning the trophies that Celtic fans expect of the team - will surely help the youngster realise the enormous potential Grimmer believes he has.