Manchester United expect to be without Marcos Rojo for some time despite the defender's claim his shoulder injury is not serious.
The Argentinian was carried off on a stretcher in the second half of United's 1-0 derby defeat to City at the Etihad Stadium on Sunday after he mis-judged a sliding tackle and dislocated his left shoulder.
The versatile defender seemed to be in some pain when he was carried off after receiving around two minutes' worth of treatment.
It was something of a surprise, therefore, to see the 24-year-old play down the injury on Twitter the day after the derby.
"Gracias a todos por los mensajes. Gracias a dios fue solo un mal momento/ thank you all for your messages. thank god it was nothing," Rojo wrote.
United believe Rojo, who speaks only limited English as he has only been in the country for just over two months, made something of a mistake in his translation of the Spanish part of his tweet.
"Solo un mal momento" translates as "only a bad moment", rather than "nothing", and the club believe the defender's injury is serious.
Rojo was discharged from hospital on Sunday evening and the club confirmed that the defender had suffered a dislocation. Medical estimates of a recovery time from such an injury range from six weeks to three months.
In 2011 United defender Rafael was out for three-and-a-half months with a dislocated shoulder. Although Rojo is yet to convince many he is the answer to United's defensive problems, his absence is a blow to Louis van Gaal.
The United manager is set to be without Phil Jones for some time as the England centre-back is suffering from shin splints - his second injury of the season.
Meanwhile, Marouane Fellaini will face no action from the English Football Association after he was cleared of intentionally spitting at Sergio Aguero in Sunday's Manchester derby.
United midfielder Fellaini and striker Aguero clashed in the first half of City's 1-0 victory at the Etihad Stadium on Sunday. Aguero tumbled to the floor after Fellaini kicked his standing leg and the hosts appealed for a penalty, which was not given.
TV replays showed that some spit appeared to exit the Belgian's mouth as he unleashed his tirade at the striker, but after reviewing the incident the FA's disciplinary panel believe it was not intentional.