Ever since I put my new budgie in the cage with my old budgie the old budgie is standing on one foot.?

About a week ago I had 2 budgies and took them to the vet because the one was exhibiting signs of illness. Vet said she had pneumonia, gave me antiobiotics etc. Unfortunately, the poor birdie died the same day. During the appt. the vet advised me if this one dies to get another one for my remaining bird. So we waited a day and purchased a new little guy last night. The vet didn't advise putting them in separate cages or anything so we did not do that...but ever since we have put the new bird in with the old the new bird seems terrified (breathing really heavy and not moving around a bunch) and the old bird is standing right in front of it on one foot. The old bird is singing to the new bird and doesn't seem to be in any pain otherwise but I've never seen him stand on one foot before and it's been the same foot each time I've looked. I know I've read it's normal behavior when they are relaxed but I'm worried that the other foot may be hurt...Any advice would be greatly appreciated.