Vladimir Guerrero Jr. Drills Young Home Run Derby Outfielder In Head With Line Drive

A fun day at the ballpark turned into a nightmare for one Home Run Derby volunteer on Monday night ... because as he was trying to help shag balls at the MLB's pre-All-Star Game event, he got drilled in the face by a Vladimir Guerrero Jr. line drive.

The scary incident happened during the final round of the showcase at T-Mobile Park in Seattle ... when Guerrero Jr. was trying to put up a big HR total to beat out Tampa Bay Rays outfielder Randy Arozarena for the event's trophy.

The Toronto Blue Jays star came up hacking ... clearing the wall with one swing -- but on his next attempt, he got a little on top of the ball ... and smoked a 115 MPH frozen rope to left field.

One of the many younger aids who the league brought in to help corral baseballs attempted to catch it on the fly -- but it missed his glove, and hit him right in the forehead.

Immediately, the guy crumpled to the ground in pain.

One of the many spectators in the crowd told TMZ Sports ... trainers raced to tend to the young man, wrapping his head in a bandage before giving him what appeared to be an ice pack.

Further details regarding the status of the injured volunteer are unclear, however ... we've reached out to MLB officials for comment, but so far, no word back yet.