The 20-year spent the last 18 months at Parkhead where he scored 17 goals in 60 appearances.
Roberts has signed a season-long loan deal this time around as the Bhoys prepare for another crack at the Champions League.
“I watched it [the draw] in Manchester with all the lads there, but I had an eye on Celtic to see who they got,” said Roberts.
“It’s a hard draw but it’s good, and it’s where I strive to play in, while we know what this club can bring to the Champions League. I haven’t seen the Celtic lads yet but I’m sure they will all be looking forward to it and will be raring to go when it gets underway.
“Celtic Park was rocking last year in the Champions League and I’m sure it will be the exact same this year. I would love to be out there playing for the fans in that environment and entertaining them.”