The £62 million ($75.5m) signing from Brighton made his second appearance of the season in Wednesday's 1-0 Carabao Cup fourth-round win against his former club but is yet to play a single Premier League minute since Mauricio Pochettino became head coach in July.
Cucurella told ESPN as part of an exclusive interview in July that he wanted to stay at the club and fight for his place but a lack of regular minutes in the coming months will likely force a rethink.
FIFA regulations stipulate that a player cannot represent three clubs in a single season and given United wanted to insert a break clause in any deal to allow them to return Cucurella to Chelsea, talks broke down.
Reports in Spain have linked Real Madrid with a possible loan bid in January and the prospect of returning to the country of his birth is thought to appeal to Cucurella. United are unlikely to re-enter the running given they signed Sergio Reguilón on loan from Tottenham after switching their attentions from Cucurella.
Chelsea would prefer a permanent transfer to help balance the books after another summer of spending which took Chelsea's total outlay on transfers past the £1 billion mark since Todd Boehly and Clearlake Capital completed their takeover of the club last May.