Over the last few years, the MLS talent factory has kicked into overdrive.
Slonina certainly won't be the last to cross the pond this year and, indeed, he's only the latest in an increasingly long line of young Americans to do so.
In fact, the list is getting so long that it got us thinking about a combined XI of Americans to leave MLS for Europe. Here's one looking back from the 2019 season:
Watch the story of Charlotte FC's journey to MLS in 90min's The Making of Charlotte FC, presented by DoorDash, on 90min channels now. Subscribe to our new US YouTube channel.
1. GK: Matt Turner
Though he was a later bloomer than most, Matt Turner has finally earned his big move, joining Premier League giants Arsenal this summer.
That self-confessed 'dream' transfer came about after a 2021 season that saw Turner win MLS Goalkeeper of the Year and Best XI honors, as well as the Supporters' Shield with the New England Revolution.
At international level, Turner also won the Golden Glove award and was named in the Best XI as the USMNT lifted the Concacaf Gold Cup. He'll now compete with the likes of Zack Steffen, Slonina and Sean Johnson for the No.1 jersey at the World Cup later this year.
2. RB: Joe Scally
Joe Scally only actually played seven senior games for NYCFC, but they certainly had a big hand in his development with three years in the club's youth system prior to his debut.
Nowadays, Scally is a regular starter in the German Bundesliga with Borussia Monchengladbach, while he finally got his USMNT break this year.
Getting regular minutes at a high standard, Scally has to go to Qatar.
3. CB: Chris Richards
While Scally only played a handful of MLS games, Chris Richards didn't play any. But again, FC Dallas deserve huge credit in helping along his development and assisting a move to Bayern Munich.
Richards enjoyed some early success with Bayern before two solid loan spells with Hoffenheim. This summer, the 22-year-old joined Premier League side Crystal Palace where he'll try to win back his spot on the USMNT.
4. CB: Mark McKenzie
Mark McKenzie has endured a mixed time since moving to Genk in January 2021, coming in and out of the team.
However, he's still only 23 and has clear talent both as an out-and-out defender and with the ball at his feet. McKenzie is capped eight times for the United States so far and is only a good run of form away from adding to that total.
5. LB: George Bello
George Bello likely would have been in Europe much sooner had injury not derailed his 2019 season. But he soon got back on the horse and became a key player not only for Atlanta United, but for the USMNT during their Gold Cup win last year.
That was enough to earn him a move to Arminia Bielefeld last January and although they suffered relegation to the 2. Bundesliga, that could be a great spot for Bello to develop and thrive this season.
6. CM: Gianluca Busio
Gianluca Busio always had talent but he really came to life toward the end of the 2020 season, scoring in the Playoffs and starting 2021 like a house on fire for Sporting KC.
The midfielder got his European switch with Venezia, then a Serie A team but having since suffered relegation.
Busio is a talented ball player and hard-working out of possession. He really should thrive in Serie B before stepping back up a level for club and country.
7. CM: James Sands
James Sands' last act for NYCFC was helping deliver the MLS Cup trophy - a fitting end for the club's first-ever homegrown player.
The 22-year-old - capable of playing both in midfield and as a center-back - is now finding his way with Scottish giants Rangers, who he helped reach the Europa League final last season.
8. CM: Tyler Adams
The pick of the bunch so far, it feels like a lifetime ago since Tyler Adams left the New York Red Bulls for RB Leipzig but, in reality, it's only been three years.
During that time, Adams has become a leader and key player for the USMNT and excelled in Germany before earning a big move to the Premier League with Leeds United. He reunites with former RBNY head coach Jesse Marsch at Elland Road.
9. RW: Brenden Aaronson
Adams' move to Leeds also sees him link up with USMNT teammate Brenden Aaronson, who is one of the Stars and Stripes' most exciting prospects.
Aaronson enjoyed a wonderful breakthrough at the Philadelphia Union alongside McKenzie, helping the club in the 2020 Supporters' Shield.
The playmaker then switched to Red Bull Salzburg, where he scored 13 goals in 66 games across two seasons, winning the domestic double in each. Combine that with six goals in 22 USMNT caps and you can see why Leeds paid over $30m for Aaronson.
10. ST: Ricardo Pepi
At least in terms of the initial fee paid, Ricardo Pepi is the most expensive homegrown player in MLS history with Augsburg parting with $20m for his services in January - exclusively reported by 90min.
The 19-year-old is still adjusting to life in the German top flight, but nobody is forgetting his 16-goal/4-assist spell with FC Dallas, nor how he grabbed three goals and two assists across his first two caps for the United States.
Pepi has huge potential and a good start to this season with Augsburg will soon have him back in the USMNT conversation.
11. LW: Kevin Paredes
Kevin Paredes will likely see most of his minutes at full-back or wing-back for Wolfsburg but in reality, he can play anywhere down the left flank.
The German giants paid in excess of $7m for Paredes, who is still only 19 years old, but showed immense talent both as an attacker and defender during a 41-game spell with D.C. United that yielded three goals and four assists.
This really could be Paredes' breakout year in Europe.
Honorable mentions: Zack Steffen, Reggie Cannon, Bryan Reynolds, Justin Che, Tanner Tessmann, Auston Trusty, Sam Vines, Cole Bassett, Jack de Vries, Daryl Dike.