Buttner was slightly fortunate, although he did surge past three players to get in position to let fly with a shot that Al Habsi should have kept out.
There was nothing lucky about Powell's effort though as the 18-year-old belted home from 20 yards.
"Nick Powell is going to be a really good player," said Ferguson. "He has a great temperament. He has two feet and is quick. He is a great striker of the ball.
"For an 18-year-old boy it has been a terrific day.
"Alexander was a little bit rash at times but he has a great engine. He is raw and has a lot of rough edges. Hopefully we can develop that and he will be a really good addition for us."