Back from a three-game rehabilitation assignment with triple-A Oklahoma City,
"It looks like he's hitting BP [batting practice]," Grandal said. "At least he did when I was there."
Grandal, who is expected to be activated Saturday from the seven-day concussion disabled list, was with Oklahoma City on Thursday when Seager was six for six with two doubles, a home run and six runs batted in.
"I think his end-game swings for base hits were better than if he were taking BP out on the field," Grandal said. "It was pretty impressive."
Seager, a 21-year-old shortstop, is widely considered the No. 1 prospect in the Dodgers farm system.
"He wasn't missing a pitch down there, no matter if you threw it lefty or righty," Grandal said. "It didn't matter. Really impressed by him. He's got really good hands at short, too. He made a couple of good plays up the middle. But the bat really impressed me. Not too many guys go six for six and miss a cycle because he hit the ball too hard."
Seager hit a ground-rule double in his final at-bat Thursday.
"It was a ground-rule double, but it could have been an easy triple for him," Grandal said. "They were kind of playing him to left field because there was a lefty throwing," Grandal said. "Just pulled it down the right-field line. The fence comes in to about 315, so the ball just bounced over."
Seager was 13 for 18 in the four-game series at Salt Lake, raising his average from .238 to .324 in the process.
Mattingly said he thinks Seager could play in the major leagues now, but reiterated that the organization doesn't plan to fast-track him.