The Reds hold a 2-0 lead over Montreal Impact following their home win in Toronto last week but will be without Richard Eckersley, Terry Dunfield, Darel Russell, Hogan Ephraim and maybe even Justin Braun for the return bout at Saputo Stadium.
Despite the injury worries and the 2-1 MLS loss to New York Red Bull on the weekend, Nelsen said the players were going in with confidence against an Impact side fresh off a 2-0 victory over Chicago Fire.
"At the moment, I don't think (Ephraim will be available), as well as several others," Nelsen told reporters on Tuesday.
"They are all knocks. That's the unfortunate thing about it. They all came in things like collisions. That is also a good sign, because they will all get cleared up pretty quickly.
"(Dunfield) is out jogging now, which is a good start. I think he will progress every day and hopefully he can be out training pretty soon. Darel is kind of back and is doing light training. I think he should be available when we go out west. Ecks is still a bit off."
He added: "Montreal will be coming out all guns blazing. They will play their strongest team possible and they will have their fans right behind them. I'm really confident in the guys we've got. They played really well in the first leg. We know that it will be a lot of tougher up there, but it will be a really good test for this young squad."