The 21-year-old will return to parent club Blackburn Rovers after the Reds’ weekend home game with Preston with Rovers forced to bring him back after his initial 28-day stint at Oakwell due to several injuries at Ewood Park.
The news represents a disappointment to Johnson, with O’Sullivan having provided a pacy option during his time with the Reds. He did not finish on the losing side in seven appearances.
Johnson says he is scouring through a list of alternative loan targets, while being mindful that if he replaces O’Sullivan, it will again mean he will have six loan players on the books, with only five allowed to be included in a match-day squad.
Despite those factors, Johnson admits he is likely to bring in a like-for-like replacement for O’Sullivan.
Johnson, whose side now embark on the first leg of a tough triple-header against the top five sides in League One – starting with Preston tomorrow – said: “I think I will as I think, in an ideal world, that we need someone to replace John.
“He’s done very well, but it is an opportunity to tinker and bring someone in if it is right.
“If it is not, then we won’t because there is no point just bringing someone in to get them fit for another club.
“They have to be ready and raring to go.
“But we are certainly on the case. We have got a decent list of names and you do all the due diligence you can to find out about who everyone is and whether they are match-fit and mentally fit and raring to go.
“I would like to bring in somebody with a bit of speed and be an out-ball because that is what John has given us. Hopefully someone with a bit of quality as well and if they have got a goal in them, even better.
“We are chipping away and are always scouring the market and there’s constant evolution at a football club.
“You have a certain list of loan players and permanent (signings) and people who can possibly turn loans into a permanent.
“You are in touch with agents from different clubs almost every day.”