The Saints paid a fee reported to be close to £12m for the Republic of Ireland international, and Bruce believes that it turned out to be a good deal for both clubs.
"I was a little bit surprised [when Long left]," the Yorkshire Post quote Bruce as saying. "He is a good player and Southampton paid a lot of money for him.
"Obviously, we felt the price was right to let him go and I think it was a good opportunity for Shane. It was good all round because he has gone there and done well. I am sure he's enjoying it.
"Shane did do a good job for us. He is a good player and we know his strengths. He's very quick, very direct. He did well in our game against West Brom, his old club, when he first came here, so I'm sure he'll be looking to do the same again. Hopefully, we can stop him."