"He feels a lot of pressure on him," Arizona manager Kirk Gibson said. "I know the fans were booing yesterday, that was kind of a first. That worries me a little bit."
It's been a frustrating season thus far for Upton, who injured his thumb early in the year and then struggled to find consistency at the plate.
Tuesday, the Mariners elected to intentionally walk hot-hitting Aaron Hill to load the bases for Upton in the fifth inning, and he struck out swinging.
Gibson cringed when he heard the boos, remembering when he heard boos as a player in 1983 from his home crowd at Tiger Stadium.
"First of all, it's not going to help him," Gibson said. "But at the same time, it introduces a new dimension into his growth as a player. He's got to remain determined, and he has to power through that. It's something different. It hurts. There's nobody trying harder than Justin Upton. It will be a good test of his character and his ability to get through this. You're motivated all the time, so if someone boos you, you don't get more motivated. It's something where you have to not let it upset you."