Forrest was forced off after 69 minutes with a hamstring injury while Matthews had to be replaced earlier by Efe Ambrose after injuring a calf in the warm-up.
Celtic boss Ronny Deila said of the injuries: "I hope it is not serious but it didn't seem good when [Forrest] walked off the pitch. I will see but I think [he will miss out].
"There was something with [Matthews's] calf and we need to switch before we started. Efe played very well at full back but we will see how big an injury it is."
Celtic face Slovenian side Maribor in the first leg of the Champions League qualifier on Wednesday night.