Johnson wasn't in the starting 11 because of a hamstring tweak he picked up in training earlier this week, but he entered the match in the 68th minute for Williams.
Despite the fresh legs of Williams and Robbie Findley entering simultaneously, Salt Lake grew increasingly more fatigued and the players' inability to slow things down earlier started to catch up with them.
FC Dallas' desperate attack finally paid off in the 88th minute as Eric Avila scored a wonderful goal less than a minute after coming on as a substitute.
MLS MVP finalist David Ferreira was the instigator of the goal, as he fought off several challenges and foul attempts in the midfield.
"I was behind, so I couldn't really crush him," said defender Chris Wingert. "I tried to get enough of him, I tried to hold him up twice. But credit to him, he stayed up."
Wisely the referee let the advantage play out and Avila made the most of it.
"Ferreira made the whole play, he's got two or three of our players draped all over him and none of us made the game-saving tackle on him or perhaps a game-saving foul," said Kreis.
Now RSL returns home needing a win to have any hope of extending its season.
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