Winger Costa, who joined the Midlands side in a £13m deal in January, earned plaudits for his breakout season in English football and was linked with the likes of Liverpool, Arsenal and Chelsea earlier this summer.
The 23-year-old was forced to undergo surgery on a long-term ankle problem last month, however, with the initial timeframe on his recovery put at three months.
"He is a couple of weeks," Nuno revealed today. "He's still on recovery, hopefully in a couple of weeks he'll join the group and then it's all about getting back in shape again... hard work for him.
"Then we'll choose the right moment because after he joins he cannot take a step back. He has to join [training] and progress until he's ready to be an option for us."
Meanwhile, Marshall - a January addition from Blackburn Rovers - has been out since picking up a hip injury during Wolves' pre-season tour of Austria.
"Just a few weeks for Ben as well," Nuno added. "He's the same process. But it's important we have a squad to choose from and options to make. When they join the group, for certain we're going to be strong."
Nuno's side got off to the perfect start in the league last weekend with a 1-0 victory over division favourites Middlesbrough and next travel to another of the promotion contenders, Derby County, tomorrow afternoon.