The 32-year old German goalkeeper played alongside the 24-year old Japan ace at Dortmund prior to Kagawa's £17million move in the summer and he believes he has plenty more to show after struggling with injuries in his debut campaign at Old Trafford.
"Shinji's difficult season is probably related to the injury he had, but now he's fit and back to his best form he can show what he can do," said Weidenfeller.
"He's proven he has a lot of potential and only a few weeks ago he scored three goals, so he is getting better all the time. He's still young, so there's a lot more to be expected of him and he will develop.
"Shinji is a really good techical player, he has great technical skills and another great advantage of his as a player is that what he does is unexpected, so opponents find it hard to foresee what he's going to do, as he's so agile and moves quickly around the pitch.
"Another advantage Kagawa is that he is flexible in being able to play in different positions. At Dortmund he always played just off the main striker, but I wouldn't necessarily tie him down to just one position."