McKenna was making his second career NHL start and third appearance overall.
Roberts, playing in his second game after missing 28 games with a left elbow injury, redirected Steve Eminger's shot with 6:26 left.
The Lightning stopped a four-game losing streak.
Yann Danis stopped 27 shots for the Islanders, who have lost two in a row following a four-game winning streak. Danis won his previous four starts.
Tampa Bay starting goalie Mike Smith missed his third straight game because of an undisclosed upper-body injury. Lightning general manager Brian Lawton said Smith is expected to practice by Monday and could play soon after.
"He's progressing nicely," Lawton said.
Danis made several strong saves, including an in-close chance by Martin St. Louis, when Tampa Bay kept the puck in the Islanders' zone for the entire 2 minutes of a first-period power play.
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McKenna stopped a right-circle shot by Trent Hunter and then used his stick to keep the loose puck from going into the net in the second.
Tampa Bay defenseman Matt Smaby and Islanders enforcer Tim Jackman fought 4 seconds into the game. Jackman and Lightning star Vincent Lecavalier had a fight during New York's 3-1 win over the Lightning on Tuesday night.
