Manchester United simply have to spend in January if they still harbour any ambitions of catching Manchester City.

The gap at the top of the Premier League stands at a huge - and probably insurmountable - 15 points.

It goes without saying that a new left-back is on Jose Mourinho's agenda, despite Luke Shaw having featured in the last three games against Everton, Southampton, and Burnley.

Matteo Darmain, Marcos Rojo, Daley Blind, and even Ashley Young can also operate in the Red Devils' problem position, but understandably, Mourinho would prefer a more regular option.

Ed Woodward clearly agrees, as several reliable sources suggest that they have begun to identify targets.

Writing for the Daily Record, journalist Duncan Castles who has a good grip on United reports that they are monitoring Celtic's Kieran Tierney, with Fulham's 17-year-old starlet Ryan Sessegnon a further option.

Both are rated as exceptional talents and the Hoops defender has made nine appearances at international level with Scotland. Sessegnon, meanwhile, has proven his ability going forward, finding the back of the net seven times in the Championship this season.

That should be music to Mourinho's ears. So, what's the problem?

Mourinho wants experience 

According to the Manchester Evening News, the Portuguese saw Tottenham's Danny Rose as his 'preferred option', with the England international having hinted at his desire to leave in the summer in an interview with the Sun.

United were put off by his potential price tag, particularly as the last time Spurs let one of their full-backs go, Kyle Walker cost City £50m.

Club officials therefore see Tierney and Sessegnon as better investments in the long-term.

Castles adds that Mourinho wanted a more experienced defender, such as Juventus' Alex Sandro - again, though, the Bianconeri's asking price of €70m is a major stumbling block.

Frustrating though it may be, the vast majority of clubs are going to struggle to compete with City, not just on the pitch, but financially too.

On the bright side, at least Shaw continues to improve and should either the Tierney or Sessegnon deals happen, the youngsters both have bags of potential.

Which left-back should United sign? Have your say in the comments.