"I could never imagine multiple people sitting down during the national anthem on the Sept. 11th anniversary," Upton wrote.
"The lessons of 9/11 should teach us that if we come together, the world can be a better and more peaceful place."
Upton was then forced to defend her comments on social media.
As reported by The New York Post, baseball has not followed the lead of the rogue NFL players.
Ever since NFL quarterback Kaepernick sat during the national anthem before a pre-season game on August 26, athletes from women's soccer to high school football have paid tribute to Kaepernick and his stand for racial equality by sitting or kneeling in protests of their own.
So why haven't we seen any MLB players follow suit?
"Baseball is a white man's sport," Orioles centre fielder Adam Jones told USA Today.
Jones said the relatively low (and dwindling) number of African-American players in the league - 8 per cent, compared to 68 per cent in the NFL and 74 per cent in the NBA - makes it more challenging and uncomfortable to speak up on racial issues.
"We already have two strikes against us already, so you might as well not kick yourself out of the game," Jones said.
"In football, you can't kick them out. You need those players. In baseball, they don't need us."
Jones recently appeared to suggest the Mets' minor-league deal with ex-NFLer Tim Tebow last week was another instance of prejudice in baseball.
In the revealing interview, Jones lamented how outspokenness is perceived differently depending on race ("It's crazy how when people of colour speak up, we're always ridiculed. But when people that are not of color speak up, it's their right") and said he feels the consequences of sharing his political views as an athlete are exacerbated.
"The outside world doesn't really respect athletes unless they talk about what they want them to talk about," Jones told the newspaper.
"Society doesn't think we deserve the right to have an opinion on social issues. We make a lot of money, so we just have to talk baseball, talk football."
Jones, who says he will continue to support Kaepernick and other athletes who stand up for the rights of minorities, isn't closing the door on a demonstration on the baseball diamond.
"No one has done it, yet," Jones says. "But that's the key word here: yet."