Guus Hiddink will be watching Chelsea's performance from the stands [AFP]

Now the London club's under-performing players come under the microscope on Saturday at second division Watford in the FA Cup, fifth round.

English Premier League leaders Manchester United continue their quest for five trophies this season at Derby County on Sunday while Swansea City and Cardiff City, both of whom have high hopes of top flight football next season, fly the flag for Wales.

Swansea host Fulham while Cardiff travel to Arsenal, the side they beat to become the only Welsh winners of the silverware in 1927, in a snow delayed fourth-round replay after a the original tie ended 0-0.

Two of the Premier League's in-form sides also meet for a quarter-final spot as sixth-placed Everton take on third-placed Aston Villa at Goodison Park.

Dutchman Guus Hiddink, named on Tuesday as Scolari's replacement as Chelsea coach until the end of the season, will be at Vicarage Road for a match that will tell him a great deal about the mood of the side he has inherited.

Scolari's exit prompted newspaper reports of dressing room disharmony at Chelsea with captain John Terry saying that the players needed to take their share of the blame for a season that would reach crisis proportions with defeat at Watford.

Second-tier challenge

Chelsea have already been eliminated from the League Cup by second-tier opposition this season when Burnley beat them on penalties at Stamford Bridge.

Another embarrassing exit is unthinkable for a club whose Premier League title hopes are fading fast and who face a daunting Champions League test against Juventus.

The mood could not be more different at Manchester United who look unstoppable in the Premier League.

They will be wary of another trip to Pride Park, however, scene of their last defeat in all competitions when they suffered a shock first leg reverse in the League Cup semi-final.

Alex Ferguson's side have a huge workload on the horizon as they challenge for honours on all fronts and the last thing they need is another match crammed into their schedule so will be looking for a no-fuss victory at Pride Park on Sunday.

Jason Scotland in action during Swansea's fourth round clash against Portsmouth [GALLO/GETTY]

Swansea accounted for holders Portsmouth in the stand-out performance of the fourth round and there is a wave of optimism sweeping through the valleys as Roberto Martinez's side prepare to host a Fulham team that struggles away.

With Martinez in charge and several other Spaniards in the squad, the Swans have caught the eye this season and defender Angel Rangel is relishing the tie.

"If we can keep playing as well as we have been I believe we can get a positive result," he told the club's website.

"They [the other Spanish players] didn't know too much about it [FA Cup] before they came.

"But now they do and, just like everybody else, they know that the FA Cup is one of the biggest competitions in the world."

In other fifth round ties West Ham United manager Gianfranco Zola, a winner of the competition as a player with Chelsea, faces a home tie against Premier League rivals Middlesbrough while struggling Blackburn Rovers host Coventry City.

Hull City face a Yorkshire derby at Sheffield United.