The Tykes were beaten at Nottingham Forest in midweek after conceding a late goal but Mark Robins' side had won their previous three games before the trip to the City Ground.
Although the south Yorkshire club are languishing just above the relegation zone, Johnson knows his players must be on their guard.
He told the club's official website: "They were unlucky at Forest, conceding a late goal.
"They are a good footballing side, different to the likes of Doncaster and Swansea in that they play more directly to those sides, but they play good football.
"They have got players who can win games, so must go there and be solid."