Manchester United completed the signing of Alexis Sanchez in January and former Arsenal midfielder Paul Merson has backed the Red Devils to further strengthen the squad in the summer in order to close the gap with Manchester City.

The Chilean international left Arsenal and moved to Old Trafford on 22 January. As a part of the deal, the 20-time champions of England allowed Henrikh Mkhitaryan move in the opposite direction.

Merson claims Sanchez's arrival will "give them X-factor" and hailed United for adding the former Barcelona forward to their squad.

"Sanchez is moving up to the biggest club in world football in my opinion. It's a great signing for United, his arrival gives them X-factor," Merson told Sky Sports.

"They will go in the market again next summer and it will game on next season. If I was a United fan I'd be looking forward to it."

Jose Mourinho has admitted that his new signing could make his United debut when they make a trip to Huish Park to face Yeovil Town in the third round of the FA Cup on Friday (26 January). It is still not sure if Sanchez will be included in the starting XI or will he come off the bench.

Sanchez's signing has been well received by United fans and a majority of their supporters' joy was further extended when Mourinho committed his future to the club. There were reports suggesting the Portuguese tactician would leave his current employers after the end of the season.

IBTimes UK confirmed on 16 January that Mourinho will commit his future to United. He has now put pen to paper on a new contract that will see him stay at Old Trafford at least until 2020, with an option for a further year.

Merson claims Mourinho's decision to extend his contract suggests the former Real Madrid and Chelsea manager is confident that his side can catch Pep Guardiola's side.

"If you're Manchester United fan, Mourinho's extension sends out a message that they are going for Manchester City," he said.

"This gives a message to the fans that Mourinho thinks he can catch Manchester City. I don't think he would be there if he didn't think he could catch them.

"Without a doubt, Mourinho sees this as one of the biggest challenges of his career - I don't think there is another challenge in Europe at the moment like this, chasing down Man City who are one of the best teams in world football.

"I think Mourinho believes he can catch them, otherwise he would go and say there was no point."

Jose Mourinho
Jose Mourinho's contract at Manchester United will now run down in 2020 with an option to further extend it by a year Getty