Now, however, with a chance to play on a much grander stage, Washington is hopeful that its young outfield prospect will show everyone else what it has known all along. Diaz will be participating in the XM Satellite Radio Futures Games in Detroit -- and if the game ultimately serves as his coming-out party, the Nats would be thrilled.
Diaz, 21, has started to hit his stride playing at Potomac of the Class A Carolina League. He's hitting .305 through 78 games with nine homers and 38 RBIs. While none of those numbers are tops on the team, his 97 hits over the same span are 18 hits better than teammate Kory Casto.
"I wasn't surprised that he was named to the Futures roster," Washington director of player development Adam Wogan said. "I was real happy for him. I do think it was a great jump for a guy who hasn't gotten much attention and hasn't had a breakout year. This is the ultimate stage for a prospect and he hasn't gotten attention on that kind of stage.
"I would love it [if he had a big game]. He's been on fire lately. He's done this at a higher level, though, playing in Winter Ball. He's shown there that he can step up against quality pitching. So if he goes there [to Detroit] and gets the attention, that's great. It will be a great thing for him to go there against those guys and challenge himself."
Diaz has been rising to challenges since the Nats signed him in 2001 as a 17-year-old. He's made a steady rise through the Washington system, spending almost a full season in the Gulf Coast League over two years, a full year in the South Atlantic League and just about full season in the Florida State League.
While the path he's taken hasn't always been easy -- he struggled a bit last season making the jump from the Sally League to the FSL -- Diaz has shown an ability to adapt and grow. The Nats are hoping that playing in the Futures Game will help continuing that process.
"He was a fairly raw hitter at the time he signed and he's just been able to develop through getting a number of at-bats," Wogan said. "Prototypically, he would be the ideal on-base guy. He doesn't strike out much, he's a good line-drive hitter and we think he'll develop more power. He really thrives in the three hole.
"He may not walk 20 times this year but for us, that's okay because he doesn't swing and miss a lot. If he sees a pitch he likes, he doesn't let it go by. So I don't know if he'd be the ideal leadoff guy."
The Nats expect Diaz to spend the entire season in the Carolina League, where he's getting the chance to play center field every day. While he doesn't have the speed and cover the kind of ground of a traditional center fielder, Diaz does have a strong arm and has not been overmatched at the position.
"Whenever Frank has played there, he's done very well," Wogan said. "He's not a burner or a plus runner but he has a strong arm and that's an asset in center field. Now whether he winds up in center field or right field, I don't know. But he can play both and play both well. And for now, that's a choice we'll make down the road."
By staying in the Carolina League, Diaz will also have a chance to go through league pitching a second and third time, giving Washington a chance to see how he adapts once the opposition has had a chance to study him. The club is interested to see if he is ahead of the pitchers or if they are ahead of him in terms of development.
And if the feeling the club has had all along about Diaz is any indication of what to expect, don't be surprised if he comes out on top.
Kevin Czerwinski is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.