The 22-year-old had been with the Bhoys since 2005 before completing a £25m switch to the Premier League giants on deadline day.
The left-back wrote an emotional message on social media admitting he was "heartbroken" that his relationship with some Celtic fans would never fix.
"All the lads are delighted for him", the 34-year-old said ahead of Celtic's Champions League qualifier against Cluj.
It's a sideways move if it happens, to me, and I can imagine Forrest will be hopefully staying put and looking to keep his stay at the club going.
"In the last ten minutes of the first half and the first ten or 15 of the second it was an even game and we were looking at making changes to give us the best chance to get that equalising goal".
"There are pictures on the wall all over the club of players who have left, and everyone would wish them all the best".
"He came through the youth, he has supported Celtic all the way through, he has not cost the club a penny and we have sold him for £25million".
"For me personally, it should be a fairy-tale story". He got a nice farewell from the players. "Meling is a player they know well, because Celtic has played against Rosenborg a couple of times", explained Mathisen.