Reid's absence will come as a blow to the Reds' Sky Bet Championship promotion challenge as the influential Irishman has been in outstanding form this season, scoring 10 goals in all competitions.
The 31-year-old will have the surgery this week before beginning his rehabilitation alongside fellow key midfielders Henri Lansbury and David Vaughan, who are recovering from back and knee injuries respectively.
Forest boss Billy Davies told the club's official website that they would fight to overcome their injury problems.
He said: "Unfortunately we've lost Andy to a hernia. He'll be having an operation this week but I'd expect him to be out for around four to six weeks after that.
"The number of players we are missing through injury is obviously very disappointing. But we've still got 19 or so players left and 14 matches remaining so we've got to roll our sleeves up and work as hard as we can to get the points required."
However, there is some good news on the horizon for Forest with a few players approaching full fitness.
The Scot added: "We've got one or two players back on the grass, including Eric Lichaj and Kevin Gomis who have had a couple of good days in training.
"And it's also nice to see Kelvin Wilson building up his fitness, although he is still a few weeks away."