The actor croons the song with co-star Catherine Keener and accepts it's a terrible way to remember Helm, who died in April (12).
He tells WENN, "It was horrible. I think we butchered the song. Neither of us are probably the most musical people. We love music, but talent-wise you're looking at the bottom of the talent bowl!
"That was the most nervous I've been doing a scene in a long time. We were both wrecks. Bruce insisted that we do it but I was kind of hoping that somebody would just loop it."
And Morgan admits Helm was on the set in upstate New York while they were filming: "It's sad that we just lost Levon Helm. He was in the studio when we were messing around with the song and I know that (co-star) Jane (Fonda) was hanging out with him."