Castellanos, who won the MLS Golden Boot last season, was rumored to be leaving NYCFC for much of the past 18 months. Brazilian side Palmeiras made offers for the forward but did not meet the MLS club's $15 million valuation, sources have told ESPN.
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NYCFC sporting director David Lee said the club are hoping a move to Girona -- which like New York is under the City Football Group (CFG) umbrella -- will invite transfer offers that meet or even exceed their valuation.
Lee said that there is no option for Girona -- which was promoted to La Liga ahead of this season -- to sign Castellanos on a permanent transfer.
"I think part of the reason for us making this decision is we think that by Taty performing well in Europe, it will give us the opportunity to [transfer] him at a higher valuation and a higher number than we've had so far in this transfer window," Lee said. "New York would like to retain the upside on that."
Lee added that with Girona being part of CFG, there are similarities in methodology, style of play, and how things are done off the field as well.
"I think all those things will, hopefully, allow Taty to settle quickly, and be able to make an impact as quickly as possible during the season," he said.
Castellanos showed consistent improvement since being loaned to New York midway through the 2018 campaign. After scoring once during that season and later making the loan permanent, he netted 11 times and added seven assists in 2019, and then scored six times with three assists during the pandemic-shortened 2020 campaign. His 19 goals in 2021 tied him for the top spot with D.C. United's Ola Kamara, but he claimed the Golden Boot on the basis of having more assists.
The Argentine started his professional career with Universidad de Chile in 2017 before being loaned to Montevideo City later that year. He went on to score five goals in 23 league and cup appearances for Montevideo.
At international level, Castellanos has made six appearances for Argentina's under-23 team.
"He is an unbelievable trainer, he wants to improve and get better," Lee said. "Thinking that it's just around four years since he joined, his development over those four years has been spectacular and really remarkable."
To replace Castellanos, Lee said Heber -- who was the team's starting striker prior to sustaining an ACL injury in 2020 -- along with Talles Magno will be involved at striker, though he stressed it would not be limited to those two.
"I think we've got so much depth and versatility in our forward players that I think it'll allow us to play a combination of players as a No. 9, or in any other three conditions behind the No. 9," Lee said.