Mastroianni picked up his first hit on a two-run single up the middle against Blue Jays right-hander Kyle Drabek in the fifth inning.
But when he received what he thought was the game ball from that single from Morneau after the game, his last name was misspelled and said the hit came on a dribbler against "Karl" Drabek in the wrong inning.
"He was trying to figure out what was going on," Morneau said. "He said it was the wrong pitcher, so I just threw it in the garbage. So the look on face -- he was in a little bit of shock -- it was pretty exciting. The real ball was in a glove in his locker. So that kind of made my night."
Mastroianni, who ended up going 2-for-4 with three RBIs, took the practical joke in stride, saying he'll keep both balls as keepsakes.
"That's going to stay with me, too," Mastroianni said with a laugh. "It'll be a fun little story later down the road."
Morneau said it's not a new trick, as it's something he's pulled on a few other rookies in recent years.
"I know I've done it a few times, because it's one of my favorite things to do," Morneau said. "I love doing that. I had the wrong pitcher, the wrong inning, a dribbler up the middle."