Lacey died at the age of 16 in May due to complications related to a congenital heart defect. But in her last few months of life, Napoli tried to bring her whatever joy he could.
The story starts with some coincidence. Lacey's mom, Debbie, went to high school with Napoli's mother, Donna, in Florida. When the Warner family was trying to raise money for Lacey's father, Steve, to run the 2014 Boston Marathon for Miles for Miracles, Debbie asked Donna if her son, Mike -- who plays first base for the Red Sox -- might consider helping them raise the necessary funds.
Without hesitation, Napoli provided the Warner family with the entire $4,000 needed so that Steve could run the Boston Marathon in tribute to his daughter.
But the story goes far beyond the generous check Napoli wrote.
Lacey had gone into heart failure last April, and was airlifted from her native West Virginia to Boston Children's Hospital.
One day Lacey got a visitor in her hospital room. The man happened to be her favorite baseball player.
"Mike really wanted to come see her before her surgery," said Debbie Warner. "The hospital made arrangements, and he came the day before her surgery and she thought that was the greatest thing when he walked into the room.
"He had sent so many balloons, and then he brought her all kinds of quilts for her bed, and pillows and T-shirts and hats and jerseys, everything autographed, and baseballs. It was great, it was just great. We took lots of pictures of them and he talked to her and talked to us and he was just so kind and personable.
"That was like the last fun event that Lacey really had because she never recovered from her surgery."
Napoli feels like it was the least he could do.
"I knew what was going on and just wanted to make her feel happy," Napoli said. "I heard she loved me as a player and was a huge fan and I tried to do whatever I could to make her happy. I still keep in touch with her parents and they came and saw me in Pittsburgh last season. I text with her mother all the time."
The connection between Napoli and Lacey is one the Warner family still keeps close to their heart.
"He's really a genuine person," said Debbie Warner. "He just really cares. He's such a caring person. He just takes everything to heart. Any time I see him, it's like kind of a tear in his eyes, like, happy to see us. I told his mom, Donna, I just feel like he's part of my family because of that last connection he had with Lacey."
Fittingly, before Lacey Warner got to know Napoli, she wore No. 12 while playing in a youth baseball league in West Virginia.
"She would always look for No. 12 when she watched on TV, and she called him Mikey," said Debbie Warner. "She would clap for him and we would sit here and watch the Red Sox games. We took her to the game [in 2013] when the Red Sox won the American League Championship Series, and we spent a whole bunch of time with Mike Napoli's family before the game."
The 16 years that the Warner family had with Lacey was a gift.
"They told us during her first year she wouldn't live beyond her first year, and then we finally got her to Boston [for medical attention], and we got 16 years," said Steve Warner. "They were just amazing people in Boston. We'll have a relationship with them forever, just like we will Mike. It's very special."
Napoli will be honored Dec. 2 with an award at the annual Champions for Children Dinner at Children's Hospital, not just for his kindness with Lacey Warner, but the work he has done to make a difference for many children in need.
"I just tried to put a smile on her face," said Napoli. "I like to do that. I do a lot of stuff with Boston Children's Hospital, with the kids over there. I just love kids and it's what I like to do."