Wagner died on Thursday July 30 at Scottsdale Healthcare Shea Medical Center, Arizona, reported Aceshowbiz.
"The late guitarist had undergone a coronary procedure. Then, he got much better and then worse again. It kind of went up and down a couple times. And then, the last five days, he was declining. It's still a complete shock because I'm used to him turning around," Wagner's manager said.
Cooper, who co-wrote 'Welcome to My Nightmare' and 'Only Women Bleed' among others with Wagner, released a statement on Facebook after learning of the news.
Calling Wagner one of a kind and irreplaceable, Cooper added, "There was just a magic in the way we wrote together."
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Wagner was recruited by Cooper's producer Bob Ezrin in the 1970s alongside guitarist Steve Hunter and they joined Reed's tour.
"He was always able to find exactly the right chord to match perfectly with what I was doing. I think that we always think our friends will be around as long as we are, so to hear of Dick's passing comes as a sudden shock and an enormous loss for me, Rock N Roll and to his family," Cooper added.
Wagner also fronted his own Michigan-based bands, the Frost and the Bossmen. The late artist was said to be most proud of his song 'Remember The Child', which he wrote to address the issue of child abuse.