The former West Ham United loanee attacked the ball with vigour to head home a bouncing ball after Eric Hassli's initial header deflected off Carlo Cudicini's crossbar late in the match but was immediately felled, unable to celebrate as he bled from the back of the head.
The fan who threw the can has since been arrested by police with John admitting he didn't have too much of an idea as to what had happened.
"I knew something hit me," he said. "I wasn't quite sure what happened, if I ran into the post or what. Then I looked down and saw a beer can, and I was like, that had to have hit me."
The goal was John's second of the season and helped Dallas move to the top of the Western Conference table with five wins from seven outings.