Big gulf in class here. Porto are regulars at the latter stages in European tourneys, currently bang in form, something like 20 wins on the bounce in competitive matches. CSKA Sofia are having a domestic crisis languishing way down the Bulgarian league. This is the last fixture they need. Could get messy. Odds of around 13/2 on any unquoted look generous.
Should be able to lay some off in-running for some nice green.