Reid faces three weeks out after limping out of the Reds' goalless home draw with Doncaster last weekend on his return to the starting line-up following hernia surgery.
Scans have revealed a tear in his hamstring and the influential Irishman is set for another spell on the sidelines, meaning he will miss the most eagerly-anticipated derby clash for years at the iPro Stadium on Saturday.
McClaren, who worked with Reid during his spell in charge of Forest, said: "I know Andy well and he is a big, big player for Nottingham Forest, very influential.
"So for him to be missing is a big loss for them. He has enjoyed a terrific season so far.
"But despite his loss, and the other injury problems they have, Nottingham Forest is still a very good team."
Defenders Eric Lichaj (hernia), Kelvin Wilson (back) and Jack Hobbs (ankle) and midfielders Henri Lansbury (back) and David Vaughan (knee) have all been absent through injury for Forest in recent weeks and none are expected to return in time for Saturday's clash.
In contrast, it looks like McClaren will have Will Hughes available for the game.
Hughes, recently crowned the Football League Young Player of the Year, came off during Derby's goalless draw at Reading last weekend with scans revealing a slight groin strain.
McClaren said: "He trained today (Thursday) and it looks like we will have everyone available."