Forrest picked up a hamstring injury in the latter stages of their 6-1 thrashing over Dundee United in the Scottish Premiership – a game in which Matthews was withdrawn in the warm-up with a calf problem.
Manager Ronny Deila, whose side went top of the Premiership with two wins from two, said: "I hope it is not serious but Forest didn't seem good when he walked off.
“I will see but I think he will miss out on Tuesday.
“There was something with Matthew's calf and we needed to switch before we started. Again, we will see how big an injury it is.”
Goals from Jason Denayer, Kris Commons and Stefan Johansen gave the Bhoys a 3-0 lead before Charlie Mulgrew and a brace from Jo Inge Berget confirmed a rout.
“It was a fantastic experience. I felt comfortable and prepared so it was a very good day and a very good performance,” Deila said.
“For 60 minutes it was very good with pressing and transition, the passing was much better and we created a lot of things.
“We know we have the best players in this league and we showed today that we are a very good team.
"But we can go 90 minutes instead of 60 and we have to work on that as well. In the last 10 or 15 minutes we were a little bit sleepy but I can understand it.”
Maribor v Celtic in the Champions League Qualifying play-offs is live on Sky Sports 5HD from 7pm on Wednesday.