Report: AFC Bournemouth 3-2 Cheltenham Town
Things had seemed straight forward enough for the Cherries in the first half as strikes from Sam Sherring and Gavin Kilkenny saw the hosts to a two-goal lead at half time.
However, Cheltenham roared back to draw level before Diaz's late winner to secure all three points for Bournemouth.
Youth team manager Carl Fletcher was happy with his side's performance after a tough week for his young charges.
He said: "I was pleased with the first half, we played with a good tempo in the first 45 minutes and passed the ball with speed.
"It was a tough week for the players, what with the re-arranged game at Oxford in midweek and a number of the players taking part in the under-21s game as well, a lot of players had a lot of minutes under the belts.
"That meant we played with a young side, including some under-16s and they took on board what we were trying to do with the need for leaders and players to take responsibility on the pitch.
"We dropped off in the second half as they got back into the game, but that also helps with the experience and learning.
"The subs made a difference in the second half and all-in-all I was pleased with the result and the way we went around getting it."