The number, which was unofficially kept out of circulation by the Yankees after the popular outfielder's retirement following the 2001 World Series, was worn this spring by Morgan Ensberg, a non-roster infielder who found the digits waiting for him in his Legends Field locker.
"I think a lot of people come in and think you request a number," Ensberg said. "That was certainly not the case with me. I showed up and I was No. 21. I [was] a non-roster invitee. There's not too much pull going on there."
That may have been the case, but Ensberg said the perception and reception from Grapefruit League crowds were not particularly receptive to the idea of another No. 21 roaming the Yankees' diamonds in Tampa, Fla.
"The feedback from the fans was, 'That's not your number.' It was unbelievable," Ensberg said. "The gist of it was that it was Paulie's number, and I understood."
O'Neill, now a commentator with the YES Network, obviously left deep tracks during his nine years in New York, playing a pivotal role on the four championship clubs during the Joe Torre era. But the Yankees felt the need to put the digits back in use, given the number of players in camp and no immediate plans to officially retire No. 21 in O'Neill's honor.
It was a territory that Ensberg -- an infielder who hit 36 homers with the 2005 Astros and who will back up both Jason Giambi and Alex Rodriguez this season -- did not want to infringe upon.
Ensberg said he offered fellow infielder Wilson Betemit $5,000 for No. 14, which he has worn since his high school days in Redondo Beach, Calif., but the transaction was rebuffed.
"I'm keeping it," Betemit said.
Ensberg would still like to switch to No. 14 if the opportunity arises, but for now, he has settled upon No. 11 -- worn by infielder Chris Woodward during Spring Training and last season by first baseman Doug Mientkiewicz.
Keeping O'Neill's No. 21, Ensberg said, was never a consideration once he made the team.
"I absolutely felt uncomfortable," Ensberg said. "Maybe somebody that is just a different personality could handle it, but for me, it was something out of respect. The No. 21 by itself is fine, but when you put it in the context of the Yankees, now it's almost somebody's name.
"It represents more than a number to a lot of people. I certainly don't want to step in that."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.