Middlesbrough's form has dipped since they spent big in the January transfer window.
The Spanish boss has masses of talent available to him at the Riverside Stadium, but having so much quality available can prove a challenge.
Boro have struggled to find a settled line-up in recent weeks, and Karanka admits that finding a starting XI is proving hard.
“It’s difficult but I have to manage them and I have to try to know which one is better,” Karanka said.
“The only thing that I consider is how they are. For example, Jordan didn’t play a few games and it was because I know how difficult it was for him because we played a lot of money for him and the expectation was he would score a lot of goals. For that reason I wanted to sometimes to keep him out.”
Boro were already pushing for promotion before January, but they invested heavily at the start of the year to try to ensure they claimed a top-two spot.
Middlesbrough brought in five new players in the last transfer window, with the likes of Gaston Ramirez, Jordan Rhodes and Ritchie De Laet all signing up.
However, things have turned sour in 2016 for Boro, with the squad starting to slip down the Championship table.
Middlesbrough are now in third place, and they have won just two of their last five matches.
The Teessiders are now one point behind Brighton & Hove Albion, who occupy the final automatic promotion spot, but they do have a game in hand over Chris Hughton’s side.