The elite six-team league has deservedly garnered a reputation as a "finishing school" for upper-level prospects, where the players can refine and polish their game, face similarly advanced pitching or hitting, work on new pitches or position switches, rehab from injuries that might have cost them development time or even make unofficial pro debuts after signing late in that summer's Draft.
But in some cases, players are sent as sort of a final audition as teams finalize their 40-man rosters just around the time that the fall league comes to a close.
And while several dozen players "on the bubble" to be protected got chances to convince their teams they deserved roster spots, their parent clubs knew that the players that did not get protected would not be so easy to sneak through the Rule 5 Draft, which this year will be held Thursday, Dec. 10, at the annual Winter Meetings in Indianapolis.
With the legions of scouts that attend every game in the Arizona Fall League, there is no such thing as a "sleeper" by the end of November.
In all, of the several dozen players in the AFL eligible to be added to their respective team's 40-man rosters, nearly 40 were left unprotected and are available for the Rule 5 Draft.
This isn't to say that every 40-man eligible player in Arizona was auditioning for a spot.
Many of the players added to the rosters in November were top prospects who just happened to be honing their skills in the AFL. Some of them could have gone 0-for-October or had an ERA of infinity and still would have been protected.
There were others who opened enough eyes to let their clubs know they were ready, and just as importantly to let scouts from the other 29 teams know the same thing, thus prompting teams to use up those slots to make sure they didn't lose them.
But, in the end, there were definitely players who posted great numbers, opened eyes and notebooks, and still wound up unprotected. You might see a few of them in new uniforms come Spring Training. Here are five players who may have helped their Rule 5 stocks in Arizona, even though they didn't make their own teams' 40-man rosters:
Matt McBride, Cleveland Indians, C/1B/OF: Right now it's tough to pin a defensive position on McBride, the Indians' supplemental second-round pick in 2006 out of Lehigh. A catcher by trade, he underwent shoulder surgery after the 2007 season and after missing much of 2008 had not caught again until this fall, seeing his defensive time at first base and in the outfield. But he caught 11 of his 22 games for the Peoria Saguaros, shaking off the defensive rust, including eight of his last nine games. What isn't a question is his bat. He hit .378 with four homers and 18 RBIs in those 22 games, and his 19 walks versus nine strikeouts gave him a .511 on-base average and .649 slugging percentage. During the regular season between Class A Advanced Kinston and Double-A Akron, he hit .287 with 18 homers, 99 RBIs and 44 doubles, sixth in the Minors and collected a team-record eight RBIs on July 31 for the Eastern League champion Aeros. Of the six 40-man eligible players the Indians sent to the AFL, only two made the cut -- shortstop Carlos Rivero and power-hitting Canadian outfielder Nick Weglarz. McBride could be an interesting option.
Colin Curtis, New York Yankees, OF: One of three 40-man eligible Yankee prospects in Arizona, none of whom ended up making the 40-man roster, the fourth-rounder out of ASU in 2006 has been generating buzz and with good reason. A survivor of testicular cancer who has made a comeback with two solid seasons, hitting .250 with seven homers and 48 RBIs between Double-A Trenton and Triple-A Scranton in 2009, he went on to tear it up in Arizona, hitting .397 with five homers, seven doubles and 18 RBIs in 20 games. That's a .731 slugging percentage for those playing along at home. He can play all three outfield spots.
Zach Kroenke, New York Yankees, LHP: Curtis was not the only Yankee farmhand in Arizona generating buzz. The southpaw could become another Shane Victorino, going in the Major League Rule 5 Draft twice. Taken last year by the Marlins, he didn't stick and headed back to the Yankee system where he sparkled in relief at Scranton, going 7-1 with a 1.99 ERA and limiting International League hitters to a .213 average. Though his fall league numbers won't jump out you, with a 5.28 ERA in 11 games, he fanned 14 and walked just four in 15 1/3 innings and his name is getting Rule 5 play once again.
Nevin Ashley, Tampa Bay Rays, C: Teams looking for catching depth in the Draft may be taking another look at Ashley, who hit .366 with two homers, 14 RBIs and a pair of steals in 18 games. Ashley, a sixth-round pick in 2006 out of Indiana State, had previously been known for his stellar defense, hitting just .230 between Class A Advanced Charlotte and Double-A Montgomery in 97 games in 2009. The Florida State League All-Star was named the Rays' Minor League Best Defensive Player this past season, throwing out 47.5 percent of opposing baserunners and posting a .993 fielding percentage.
Josh Perrault, Baltimore Orioles, RHP: Perrault is becoming a Rule 5 pro, but has yet to make it to the Major League phase of the draft. He's already changed organizations twice via the Minor League version, being drafted away from Arizona by Washington in 2005 and from Baltimore last year. This past season, he moved into a closer role for the first time and combined between Double-A Bowie and Triple-A Norfolk for a 2.13 ERA in 59 games, collecting 18 saves and striking out 74 while walking 18 in 72 innings, limiting hitters to a .209 average. In Arizona, he posted a 3.38 ERA in 11 games in relief.
The Rule 5 Draft will take place at 9 a.m. ET on Thursday. During the Major League phase of the Rule 5 Draft, eligible players left unprotected from their clubs' 40-man rosters may be selected for $50,000. A player selected must remain on his drafting team's active Major League roster during the following season or be sent back to the original club for $25,000. MLB.com will carry the audio portion of the event live from Indianapolis with Jonathan Mayo and Mike Siano as co-hosts.
Lisa Winston is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.