He'd have been at the finish line - where the bombings occurred.
In the Miami Heat locker room on Wednesday, Allen told reporters that a friend of his family was among those who lost a leg in Monday's deadly explosions at the marathon, and that another friend owned the restaurant that employed the 29-year-old woman who was one of the three people killed in the blasts.
"We have a family friend that was there, that ran the marathon, and she's in intensive care now because she lost her leg," Allen said Wednesday night before the Heat played host to the Orlando Magic. "A good friend of mine, one of the managers of his restaurant, the 29-year-old that was killed was his manager. And his assistant chef, I don't know if he said he lost both his legs or one, but he got hit pretty hard and he's dealing with that now."
The restaurant manager was Krystle Campbell, who had gone to the race because her best friend's boyfriend was running in the race. She worked at Jasper White's Summer Shack, according to the restaurant's website.
One of the owners of that restaurant was Patrick Lyons, a close friend of Allen's.
"I told him next year, it's going to be a great thing for you to run for her," Allen said. "I believe when he thought about it, that's a pretty good idea. I'd imagine next year, the number of people running the marathon will double."
Allen has been a spectator at the marathon several times, going back to his days with the Celtics. He said his mother has run the marathon three times and that his wife has done so twice.
"We would have been sitting at the finish line, me and my whole family, waiting on my mom and my wife. ... They would have been running if we were in Boston," said Allen, in his first season with the Heat. "So that presented some anxiety when I thought about it."
Allen said he is very familiar with the finish-area setup at the marathon, and applauded the way first responders handled the chaos.
"I'm pretty sure a lot of people as devastating as it was to them, they got care immediately, which is a critical thing," Allen said.
As several other sports teams have done in recent days, the Heat paid tribute to the victims before their game Wednesday, showing the marathon logo on their scoreboards and unfurling a massive flag over midcourt for a moment of silence.
There were some additional security measures at the Miami game Wednesday, including car searches for those vehicles that were parked in the garage underneath the arena.
"We all take it for granted," Heat coach Erik Spoelstra said. "It was such a horrific, horrifying event in Boston. You can't come to grips with it, with that type of horror."