Spain's 2010 World Cup-winning captain was passed fit to return to training after recovering from a broken finger a couple of weeks ago, but has not made the cut for their last two matches against Real Zaragoza and Galatasaray.
"This week he has improved and we are pleased to have him with us because we are entering a decisive two-month period," Karanka told a news conference ahead of Saturday's clash.
"He has achieved the competitive edge necessary and will enter the squad.
"The coaching staff look for the best for the team, and tomorrow he will not play, but in the future who knows."
Casillas's former understudy, Diego Lopez, who was brought back to the club from Sevilla in January as cover, has performed well in his absence and will retain the number one slot.
Spanish media reports of a frosty relationship between Casillas and coach Jose Mourinho have heightened the interest in the subject of when or if a player known by fans as 'Saint Iker' will return to the starting 11.
Asked if Lopez felt the interest in knowing when Casillas might return showed a lack of respect to the incumbent, Karanka replied: "He speaks out on the pitch, which is the best place to do it."
After securing a 3-0 advantage over Galatasaray in their Champions League quarter-final on Wednesday, Real are expected to rotate players with an eye on next Tuesday's return in Istanbul.
Defenders Raphael Varane and Fabio Coentrao and midfielder Sami Khedira were all left out for Saturday's game.
The champions trail leaders Barcelona by 13 points in second place with nine games left to play.
