"We didn't finalise it until a couple of hours ago," Bruce said at the press conference.
"The final decision was Matthew's and I know how difficult it is to leave a big club like Arsenal.
"He's young, he's quick, he's athletic and he's a good footballer. I believe if he didn't get injured when he did he would have forced his way into the Arsenal team.
"With the injuries we've had in that area and the money he's cost, he will go straight into the side," joked Bruce.
Upson said that the fact that the Blues boss played in the same position as he does, had a bearing on his decision to come to St. Andrew's: "Obviously it's one of the things that I've thought about.
"Hopefully he can help develop my game which is important for me.
"Mr Wenger left it quite open-ended. I think he wanted me to stay, but I think when a manager gives you an option to move I felt it was time that I moved on."
Upson has signed a three-year contract that will cost Blues an estimated initial fee of £1million which could rise to £3million depending on the club's Premiership status and appearances.