Nemanja Vidic could leave Manchester United for nothing in the summer, with the captain's agent ruling out a contract extension at the end of the season.

The defender has only six months remaining on his current deal, and although United plan to enter discussions over extending his stay at Old Trafford, the Serbian is keeping his options open. If he does leave it would be yet another blow to the manager David Moyes, who will spend the summer trying to convince Wayne Rooney to sign a new deal having entered the final 18 months of his own contract.

"Vidic, given his contract situation, has several teams looking at him," his agent Silvano Martina told Radio Crc in Napoli. "Vidic is a very good player and will have no trouble finding a new club. For the time being we are remaining calm. However, at the moment, I would rule out an extension with Manchester United."

United do not hold an option on the four-year deal Vidic signed in August 2010, unlike that inserted in to Rio Ferdinand's own 12-month terms and which is yet to be triggered. Yet the club are relaxed about the situation regarding the 32-year-old.

Despite the central defender's deal having moved into its final six months, which means a pre-contract agreement with another club could be agreed in readiness for a summer move, it is usual for United to conduct contractual talks with players in their 30s during the final few months before they would become a free agent.

While they are waiting longer before deciding whether to take up the option on Ferdinand – the 35-year-old is also seriously considering his long-term future having endured another injury-hit season that has seen the defender ruled out for another 10 days to two weeks with a knee problem – Moyes has decided that Vidic is part of his plans and is keen to enter contract talks soon. Martina's warning may simply be a negotiating tactic.

The two serious knee injuries Vidic suffered during the past three years have been followed by further niggling problems that have affected his availability. After suffering sciatica which ruled him out of the pre-season tour, Vidic was also concussed in the 1-0 win over Arsenal in early November.

He has managed 17 appearances this term.