Brandon Belt is hitting .238 with four home runs and nine RBI.
He has struck out 23 times in 80 at-bats.
"I just don't think he's hitting the fastball," Mike Krukow explained on KNBR 680 on Friday morning. "I think they're coming at him with a lot of fastballs at the belt, and until he turns some of those fastballs around, he's gonna get a continued steady diet of the same pitch.
"Go back and take a look at his home runs -- they're curveballs down and in ... this is something that's been a weakness of his for awhile. And teams are on it. They're telling him what's coming, and he's unable to hit it."
Belt has drawn 16 walks this season and his on-base percentage is .365.
"That's outstanding ... but he has to start beating that fastball," Krukow added. "And his best defense in most instances is just to take the fastball. But he can't do that. You see a belt-high fastball, a little bit above, you're thinking 'I gotta hit this.' And he's not hitting it.
"And until he starts making some adjustments, and you say, 'Well, how do you do that?' Well, you gotta flatten out your swing someway, to be able to take that loop that you have when you're swinging at that high fastball, out. And I think that's the way that you try and beat it."
"They're continuing to pound him with fastballs, and he's not doing anything with them. So it's a concern and he's got some work to do to try and solve that. He's gonna have to make some mechanical improvements."