The Academy graduate, 23, made eight appearances for the MLS club in the 2019 season, scoring once, before rejoining the club for the start of the 2020 campaign.
Sims came on as a substitute in the Red Bulls' first two matches of the season in March, before the league entered a lengthy suspension due to COVID-19.
The winger will now return to England, but remains ineligible for Saints for the remainder of 2019/20.
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