Pete Alonso signing complicates the Orioles' first base picture

With Pete Alonso officially a Baltimore Oriole, questions emerge about the futures of Ryan Mountcastle and Coby Mayo.

Nov 4, 2025; Baltimore, MD, USA; President of Baseball Operations Mike Elias introduces Craig Albernaz as the Baltimore Orioles new Manager at Warehouse Bar. Mandatory Credit: Daniel Kucin Jr.-Imagn Images | Daniel Kucin Jr.-Imagn Images

The Baltimore Orioles have shown unprecedented urgency this offseason, landing their biggest acquisition yet by signing Pete Alonso to a five-year, $155 million contract.

While Alonso is a much-needed addition for a Baltimore offense that lacked a consistent power presence last season (no Oriole hit more than 17 home runs in 2025), his arrival changes the organization's first base outlook and raises questions about the futures of Ryan Mountcastle and Coby Mayo. On top of that, the move also impacts the placement of top prospect Samuel Basallo in the lineup.

So who is most affected by Alonso's arrival?

Read More: Mike Elias reveals why Orioles appealed to Pete Alonso

Ryan Mountcastle

Sep 20, 2025; Baltimore, Maryland, USA; Baltimore Orioles first baseman Ryan Mountcastle (6) looks on during the ninth inning against the New York Yankees at Oriole Park at Camden Yards. Mandatory Credit: Daniel Kucin Jr.-Imagn Images | Daniel Kucin Jr.-Imagn Images

Mountcastle was limited to 89 games in 2025 due to a right hamstring strain and struggled to make an impact when healthy. After a strong start to his career, hitting 73 home runs across his first three full seasons, injuries and inconsistency have slowed his momentum.

The 28-year-old has now played fewer than 125 games in each of the past three seasons, and his 2025 production dipped across the board. He posted career-lows with a .286 on-base percentage, .367 slugging percentage, and .653 OPS, numbers that are cause for concern as he approaches a pivotal contract year.

Ryan Mountcastle gets all of this one and sends it into the Bird Bath 😤 pic.twitter.com/D6ai4SsKZ0

— MLB (@MLB) September 19, 2025

That said, Mountcastle was tendered a contract last month and is entering his final year of arbitration. If healthy, he could still rebound in 2026 and position himself well for free agency next offseason. However, Alonso's presence significantly limits Mountcastle's path to everyday playing time at first base.

Coby Mayo

Sep 23, 2025; Baltimore, Maryland, USA; Baltimore Orioles first baseman Coby Mayo (16) celebrates after hitting a single during the third inning against the Tampa Bay Rays at Oriole Park at Camden Yards. Mandatory Credit: Daniel Kucin Jr.-Imagn Images | Daniel Kucin Jr.-Imagn Images

Alonso's signing may have the most direct impact on Mayo, who is now increasingly seen as a trade chip.

The 24-year-old received extended opportunities at first base last season due to Mountcastle's injury, but struggled to produce consistently. Mayo finished 2025 with a .217/.299/.388 slash line, with 11 home runs and 28 RBIs. However, he did close the year on a positive note, hitting .301/.393/.548 in September.

🟠 Orioles first baseman in 2025: 14 HR, 60 RBI and slashed .243/.310/.369

🟠 Orioles DH in 2025: 23 HR, 73 RBI and slashed .222/.297/.385

🟠 Pete Alonso in 2025: 38 HR, 126 RBI and slashed .272/.347/.524

Can't wait to see this man at Camden #Birdland pic.twitter.com/4A5eJnZ0Wg

— SleeperOrioles (@SleeperOrioles) December 10, 2025

Mayo's value is in both his youth and versatility, as he came up through the minor leagues as a third baseman before fully transitioning to first base in 2025. With Alonso seemingly locked in at first base now and Jordan Westburg anchoring third base, Mayo could be included in trade discussions as the Orioles continue to pursue a frontline starter.

Read More: Orioles talking to Nationals about potential trade for star pitcher

Samuel Basallo

Sep 23, 2025; Baltimore, Maryland, USA; Baltimore Orioles designated hitter Samuel Basallo (29) winces after being hit by a pitch during the sixth inning against the Tampa Bay Rays at Oriole Park at Camden Yards. Mandatory Credit: Daniel Kucin Jr.-Imagn Images | Daniel Kucin Jr.-Imagn Images

Baltimore's top prospect isn't going anywhere, but Alonso's arrival does affect how Basallo may be used in Baltimore's lineup.

With Alonso likely assuming the role as the everyday first baseman, Basallo's chances of starting there decrease. Baltimore will want the tandem of Adley Rutschman and Basallo in the same lineup as much as possible, meaning one of them will often slot in as the designated hitter, a role that now seems more likely for Basallo moving forward.

There is no question Alonso makes Baltimore's lineup more formidable. But at the same time, his signing presents Mike Elias with decisions regarding Mountcastle and Mayo, and whether either could be used as a trade chip to further bolster Baltimore's roster.

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