The 29-year-old, who has scored 14 goals in 51 internationals, has missed Forest's last three games with a thigh injury and will not join up with his international team-mates on Monday.
Wales face Bulgaria at Cardiff City Stadium on Friday before travelling to Basel to play the Swiss the following Tuesday.
Caretaker manager Brian Flynn has called up Southampton forward Ryan Doble, 19, as his replacement.
Flynn handed Doble his first cap for the Under-21 side against Austria last season.
Flynn is already without captain Craig Bellamy for the forthcoming fixtures having left the 31-year-old out due to injury.
Bellamy has been troubled by a knee problem in recent weeks but returned to action for Cardiff on Saturday and scored in a 2-1 npower Championship win at Barnsley.