MIAMI -- The Marlins haven't fully decided on how many players will be added when rosters expand in September, but manager Don Mattingly on Sunday confirmed two rookies who project into the long-term plans of the organization will be heading to the big leagues.

Not surprisingly, outfielder Lewis Brinson and right-hander Sandy Alcantara top the list. Both could be back as early as Saturday when the callup period begins.

Brinson has been on the disabled list since July 4 with right hip inflammation, and he currently is on rehab assignment at Triple-A New Orleans.

Alcantara, who has made one big league start this season, also is at New Orleans. The 22-year-old, who is ranked by MLB Pipeline as Miami's No. 2 prospect, recently was optioned back to New Orleans after spending time on the big league disabled list with a right axillary (armpit) infection.

Brinson went 2-for-5 on Saturday for the Baby Cakes, and Alcantara started the game, giving up five runs (four earned) with four strikeouts and no walks in six innings.

Asked if Alcantara would be back in September, Mattingly said: "That's a fairly good assumption."

The Marlins don't plan on using a six-man rotation, but when Alcantara returns, they could move one of their starters to the bullpen, or give extra rest to any of the current five in the rotation.

As for Brinson, the Marlins anticipate him getting about 100 at-bats in September, where he will play center field. Rookie Magneuris Sierra will play more corner spots when Brinson returns.

"[Brinson] got a couple of hits last night, which was good to see," Mattingly said. "But, it's not always that you're looking for results. It's more about guys getting timing, getting comfortable, where you can get 12-14 at-bats and be OK. But, if you get 25-30, you should be ready. It's basically what you would get in Spring Training."

Brinson currently has 28 at-bats on his rehab assignment, which included eight games at Double-A Jacksonville before getting brought up to New Orleans.

The Marlins could have looked to bring both back sooner, but with the callup period less than a week away, they won't rush either.

Alcantara has one more scheduled Triple-A start before he will join the Marlins.

Worth noting

• First base/outfielder Garrett Cooper (right wrist sprain) has been evaluated by a hand specialist, and surgery is an option. Cooper, 27, has had an injury-plagued season in which he's played in just 14 games. He aggravated the wrist playing outfield earlier in the week on rehab assignment at Class A Advanced Jupiter.

"I'd say it's probably a possibility, but we'll see where it goes," Mattingly said when asked about surgery.

• After being off on Monday, the Marlins will open a two-game Interleague series at the Red Sox on Tuesday. Mattingly, who hit his first big league home run at Fenway Park, says playing in Boston will be a good experience for his young squad.

"That will be good for our guys, because that's a whole different environment," Mattingly said. "It's a great ballpark. It's a great environment. The fans are into it. Very knowledgeable. I think Boston will be fun. Hopefully, we'll play well."