"I feel like he's always been one of those players who can change a game or change the outcome of a game with one swing," said Kennedy, who was a teammate of Upton's for three seasons in Arizona (2010-2012).
Such was the case in the opening game of a three-game series, as Upton drilled a two-run home run to center field in the fourth inning.
For Upton, it was home run No. 20 this season and No. 15 at Petco Park, which, even though it has undergone some fence modifications in recent seasons so it would play fairer, is still regarded as pitcher-friendly.
This home run, off Reds pitcher David Holmberg, was tracked at a distance of 417 feet, according to Statcast™, with a batted-ball speed of 108 mph.
That's some serious smoke, said Padres interim manager Pat Murphy.
"Everyone who has been around Justin knows that's real power ... thoroughbred power," Murphy said.
Since July 26, Upton is hitting .280 with five home runs and 13 RBIs over those 14 games. Those home runs have had impact.
His home run against the Marlins on that day provided the difference in a 3-2 victory. Upton's three-run home run in the ninth inning on July 30 against the Mets was the game-winner.
On Monday, his home run was the difference again, as the team snapped a six-game losing skid.
"One big swing like that can totally flip the game," Kennedy said.
Upton's home run tied him with Will Venable for the most home runs in a single season at Petco Park. And consider the team still has 27 remaining home games.
Upton is in position to become only the fifth Padres player to finish over .300 at Petco Park (with at least 170 at-bats). He's currently hitting .302 in the first year playing full-time in this ballpark, which opened in 2004.
"He's picked us up over and over again," Murphy said.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.