Pinch-runner Brett Lillibridge then scored the go-ahead run with two outs on an infield single by Alexei Ramirez and Alejandro De Aza laced a two-run single as Chicago put up three unearned runs on reliever Shawn Kelley.
Saunders, sporting a big shiner under his right eye, said he simply missed what should have been a routine catch.
"I didn't take my eye off it. I wasn't running over there lazily," Saunders said. "It was just one of those things. I've been on the receiving end of a ball moving on me and catching it and just being 'oof,' but this is the first time I've ever been hit in the face.
"As an outfielder, you almost have to go out of your way to get hit in the face by a lazy fly ball like that. I feel horrible. I'm taking blame for this loss tonight. Kelley pitched his [butt] off and should have been out of there."
The White Sox were happy to take the unexpected help from a Mariners club that came in with the highest fielding percentage in the Majors. It was just the second error of the season for Saunders, who made a nice diving catch and turned it into a double play an inning earlier.
"I didn't even know what happened," said White Sox manager Robin Ventura. "You see him under it and it kind of bounces away. Baseball's weird that way."
Hernandez struggled for the fourth time in his last five starts, giving up three home runs in five innings. But Seattle's late rally got him off the hook with a no-decision after he left with a 4-0 deficit.
Manager Eric Wedge said he didn't think the back issue would force his ace to miss any time, but was enough to cause him to leave a couple pitches up in the fifth when the White Sox hit back-to-back home runs, including the second of the night by second baseman Gordon Beckham.
"I slipped on the mound making a pitch, I think in the second inning," Hernandez said. "In the fifth, I could not finish my fastball. But it wasn't the back. It was me. Just a couple mistakes. The fastball was up. The two homers Beckham hit were the same pitch.
"The ball was doing nothing. It was up. It was in the middle. It was just the fastball, because my breaking ball was pretty good. But I've got to have my fastball, too, to have a good game."
Hernandez's record remains 4-4 with a 3.42 ERA after he gave up four runs on five hits with four walks, a hit batter and six strikeouts. He's 1-3 with a 6.14 ERA his past five outings while allowing 42 hits in 29 1/3 innings.
The win was the 13th in their last 14 games for the White Sox as they improved to 30-22 and continued their recent mastery of the Mariners.
Seattle is 2-15 at U.S. Cellular Field since 2008 and 3-20 overall against the White Sox in the past three seasons, including a three-game sweep at Seattle in April that included Philip Humber's perfect game.
In their first game after putting up 21 runs on the Rangers, the Mariners managed just two runs and three hits in 6 1/3 innings against White Sox veteran Jake Peavy, but rallied to tie the game against Chicago's bullpen with a pair of runs in the top of the eighth.
For six innings, Seattle's only hit was a Justin Smoak popup in the fifth that catcher A.J. Pierzynski couldn't glove right in front of the plate. The official scorer ruled that a single for Smoak and it stood as the only hit off Peavy until Dustin Ackley lined a double down the left-field line leading off the seventh.
When second baseman Kyle Seager followed with a two-run blast to right field, the Mariners cut the lead to 4-2, and Peavy's night was done two batters later. Seager added a run-scoring single in the eighth in a 2-for-4 night with three RBIs.
Smoak tied the game with a base hit off reliever Jesse Crain before Mike Carp popped out, leaving the game tied until the White Sox rallied in the bottom of that frame.
Seager, hitting in the No. 3 position after Wedge moved Ichiro Suzuki back to his familiar leadoff role, now has six home runs and a team-leading 31 RBIs. He's gone 9-for-15 with four doubles and a home run in his past three games to raise his average to .283.
But in the end, Saunders' error was the moment that will be remembered from this one.
"This game has its good and bad," Saunders said. "Tonight certainly was the bad. I feel horrible about it. Tomorrow is a new day and I'm going to try to make up for it."