ANDREW Walker is expected to miss up to four weeks of football as Carlton's horrible luck with injury in 2012 continues.

Walker strained his right calf in the opening quarter of Friday night's game against North Melbourne and was immediately substituted off for Kane Lucas.

Blues coach Brett Ratten said he expected to lose Walker for up to a month, taking even more manpower out of a forward line that is already missing Shaun Hampson and Jarrad Waite.

"Walker's is a calf and if you're talking calves, you're probably talking 21-28 days if that was the norm. We'll get that scanned and it might be a slight strain but with Andrew I dare say it's a bit more than that," Ratten said.

Lucas got a late call-up as a replacement for Zac Tuohy, who aggravated a back injury in the warm-up after the final teams were officially named. But the Blues were given permission to make a late change to the selected team.

"Zac's had a back (injury) for a little bit and he just felt a bit sore during the week," Ratten said.

"He felt good and then after training on Thursday he felt a bit tight again and he got a bit of treatment today. It seemed like it was getting better before the game and then when he got out there he couldn't twist, turn and get low."

The Blues also lost Aaron Joseph to concussion after a collision with North Melbourne's Jack Ziebell.

Ratten said there were some encouraging signs from Marc Murphy and Lachlan Henderson, who both made their returns from injury against the Kangaroos.

The pair could be soon joined by Waite, who looks set to make his comeback via the Northern Blues after missing much of the season so far with a back injury.

"Jarrad will be back next week but he'll probably have to play VFL level. He's missed so much footy. I thought it was a pretty good effort by Marc Murphy to get out and do what he did after such a long spell," Ratten said.

"Even for Lachie, with his preparation to come out and put his hand up. Some of these players might have been rested for other periods of a season and given a little longer to get up but we're not in that position at the moment.

"We need to get our talent on the field and those players were important for us going into the game. It is frustrating but you have these things happen at some stages of the season."