Major League Baseball teams will operate alternate sites similar to those used during the pandemic-shortened 2020 season, delaying the beginning of the Triple-A season by at least a month, sources familiar with the plan told ESPN.

While Triple-A, the highest level of minor league baseball, was scheduled to begin April 6, games will be pushed back, sources said, to around the same time as Double-A, high-A and low-A are expected to start -- the first week of May.

Even then, some executives told ESPN they believe the alternate sites could last longer into the season. The reason for rekindling sites -- which serve as training facilities for players who are likeliest to be called up to the major leagues -- is the proximity to teams' home stadiums and easier oversight of testing and coronavirus protocols, according to sources. Further, Triple-A teams travel via commercial airline, whereas major league teams can go from hotel to stadium to private flights on getaway days.

Teams are hopeful the delay allows for the vaccination of players before they are sent to their minor league affiliates, which came under the management of MLB this winter. Vaccinated players would allay concerns with teams about players arriving from minor league sites and immediately joining clubs without a quarantine period.

The swift pace of vaccination across the country has heartened league and team officials, and combined with decreasing COVID-19 cases around the country, there is increased optimism about baseball's prospects for playing a full season with limited issues. Spring training games have started without a hitch, and officials were stunned at the low number of COVID-19 cases upon intake testing this spring -- 20 positive tests among more than 20,000 taken, according to the league.

Still, MLB's belief that April is likely to be the most difficult month for COVID concerns has not wavered since it tried to get the MLB Players Association to agree on a one-month delay to the season. The union rejected the idea. MLB's control of the minor leagues allows unilateral implementation of policies.

To make up for lost games in April, Triple-A teams expect their seasons to run into September -- later than the standard minor league year, which typically ends at the beginning of the month.

The number of players at each alternate site is unclear, but sources expect the sites to house about two dozen players -- a typical Triple-A roster. Last year, with a finite number of players allowed at alternate sites, teams opted for a mixture of major league-ready players and prospects whom they didn't want to lose a year of development not playing.

This incarnation of the alt site is likelier to skew older -- major league veterans and ascendant prospects who are on the cusp of the major leagues. Lower-level minor league players plan to report to spring training toward the end of March, after major league teams have vacated the facilities, and will prepare for their seasons in Arizona and Florida.