Bristol City have extended Stern John's loan spell from Championship rivals Southampton until the end of the season.
But defender Andy Webster has returned to Scottish Premier League side Rangers following his five month stay at Ashton Gate.
Striker John, who joined the Robins in October, originally signed until the end of the year. But Johnson revealed he had an informal agreement with Southampton counterpart Jan Poortvliet to keep the Trinidad and Tobago star for longer if the move worked out.
"We had a gentleman's agreement with Southampton. They have told him they won't be offering him a new contract and we need that experienced goalscorer in our squad," said Johnson.
"He is good in the dressing room and, although you won't see him rushing around on the pitch, he is a very clever player who does a lot of work for the team."
John has made seven starts and four substitute appearances for City scoring two goals.
On Webster, who made just six league and cup appearances during his time at Ashton Gate, Johnson added: "Andy was unlucky because every time we needed to push him he broke down with an injury.
"He never got up to our level of fitness, so therefore never got as many games as he came here to play."
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