The Robins managed to land the talented striker from Chelsea on a season-long loan despite a number of other clubs eager to attract his services.
However, City won the race to the 18-year-old and it has paid immediate dividends as he has scored six goals in seven Championship outings and eight in nine overall following his brace in Tuesday night's 3-2 loss to Sheffield Wednesday.
Johnson is of the belief that Abraham would be worth an absolute packet now having taken England's second division by storm.
Johnson told Tribalfootball: "We believe we have signed a top striker.
"Yes, he may be young and there will be peaks and troughs in his performances, but his enthusiasm and willingness to run are there for all to see.
"You can see his movement is good and he is electric in his pace. We've not brought him here just to sit on the bench and he will be a big part of our team and of our squad.
"We presented our philosophy to Chelsea, we promised to look after Tammy and make sure his feet stay on the ground and I was delighted to get him through the door.
"We've got ourselves a young striker whose value on the open market would be somewhere north of £20million.
"There were a lot of other clubs in for him, including foreign ones and others in the Premier League and Championship."