MILWAUKEE -- As Sept. 1 approaches, Major League teams can expand their roster to 28. For Reds fans, it’s a prime chance to wonder if this is the time that top prospect and pitcher Hunter Greene will make his arrival in the big leagues.

Greene, 22, is ranked by MLB Pipeline as the Reds' No. 1 prospect and No. 27 overall. The right-handed starting pitcher, who is at Triple-A Louisville, has routinely shown triple-digit velocity and can reach fastball speeds of up to 105 mph. His arrival would mean instant buzz for Cincinnati, and fans would likely turn out to watch him pitch at Great American Ball Park.

With the Reds experiencing bullpen problems all season, Greene would appear on the surface to be a natural fit. But it’s far from a guarantee to happen.

Let’s take a look at the situation.

1. Roster expansion is no longer up to 40 players
Rules were changed before 2021 that decreased the number of September callups teams can make. In past years, rosters could balloon from 25 players to 40. This season, teams have 26-man rosters but can activate only two additional players to bring them to 28. Gone are the days that a young prospect could get called up, spend most of his time on the bench and see action in a game or two.

Cincinnati has a few players on the injured list who could return -- namely left fielder Jesse Winker and relievers Brad Brach and Art Warren. Also possible, but less likely to return, is center fielder Nick Senzel. There would be no obvious room to carry added prospects, especially in a pennant race.

2. Greene has been just average at Triple-A
On Wednesday in a start at Indianapolis, Greene had his best start with Louisville so far. He threw five scoreless innings with two hits, two walks and three strikeouts. But in 11 starts with Louisville, he is 4-6 with a 3.78 ERA and a 1.20 WHIP. Over 52 1/3 innings, he’s walked 20 with 43 hits and struck out 67.

At Double-A Chattanooga, Greene was 5-0 with a 1.98 ERA and a 1.00 WHIP in seven starts.

3. Greene has no bullpen experience
There is concern among Reds front-office officials about whether Greene -- who underwent Tommy John surgery on his right elbow in April 2019 -- could adjust to a bullpen role on the fly after starting all season. It would require being able to warm up quickly -- sometimes multiple times in a game -- and jumping into a game. There would be few real chances to ease him into low-pressure mop-up situations.

If the Reds needed to add a reliever besides Brach or Warren, they could first turn to those with experience. Dauri Moreta is at the top of those deserving a shot. Moreta is 5-0 with a 0.83 ERA in 32 games combined at Double-A and Triple-A, including 16 2/3 scoreless innings with Louisville.

4. Greene could be needed for the rotation, this season
The Reds' rotation has been a strength of the team -- especially Wade Miley all season and Luis Castillo since June after early struggles. But there are depth issues should someone get hurt. Pitchers are working late into the year after the 2020 shortened season curtailed innings for everybody.

If someone went down, Greene could be an instant option for a promotion. Lefty Reiver Sanmartin has a 3.27 ERA in 20 games combined at Double-A and Triple-A. Another lefty, No. 2 prospect Nick Lodolo, just went on Louisville’s seven-day injured list with a left shoulder issue.

There are two experienced starters in the Reds' bullpen in Tony Santillan and Jeff Hoffman. But neither has started since July and they wouldn’t be stretched out enough to provide length in games.

5. The taxi squad is possible
Teams are also allowed to carry five players on “taxi squads” to travel and be available for immediate callups in case of injury or a COVID-19 positive test. It would be possible to add Greene here. Although he would not be active to pitch, he would get some big league exposure. It was viewed as a big help to catcher Tyler Stephenson last season when he wasn’t on the big league club and helped prepare him for what’s been a successful rookie season in 2021.