Yes — the Giants did just trade for J.J. McCarthy on September 28, sending a 2027 fifth-round pick to the Vikings (from the JJ McCarthy page). But here's the key: Jameis Winston is still the starter. The Giants explicitly said Winston is QB1 after making the trade (from the Jameis Winston page — recent news item: "Giants trade for Vikings QB McCarthy but say Winston is QB1").
The timeline makes sense: Jaxson Dart went down with a season-ending MCL injury in Week 2, Winston stepped in (though he struggled — 11-of-27, 40.7%), and the Giants brought in McCarthy as a developmental backup / fresh-start project. McCarthy was QB3 in Minnesota behind Kyler Murray and Carson Wentz and has immaturity concerns, so he's not ready to start right now.
Bottom line: Winston is the QB1 for now with McCarthy as the backup/long-term project. Fantasy-wise, Winston is a volatile mid-to-low QB2 streamer — that downfield arm gives you volume, but the turnovers are scary.
On FantasyPros, they summed it up: "Jameis Winston is anything more than a super flexible…" — the consensus is he's a stopgap, not a fantasy savior.