Diego Costa
Costa has netted nine times for Chelsea this season but his fitness has been a source of concern.Getty Images/Shaun Botterill

Chelsea have received a timely boost in the form of Diego Costa, who has been adjudged fit to start in their crucial tie against Manchester United at Old Trafford, according to the Daily Mail.

The striker has been in brilliant form this season but was feared out of this tie with a hamstring injury which has been troubling him for a few days now. The striker was also down with a virus for which he had to be hospitalised last week.

However, his dexterity in training has banished any fears of his absence with Mourinho set to extend the gap over Manchester City as they take on United in their home territory. The Red Devils, who have suffered for their leaky defence this season, will have their task cut out against Chelsea, who have proved their attacking threat over the season, dropping only two points thus far in a draw with City last month.

Meanwhile, Van Gaal has a lot of problems to worry about, especially with Wayne Rooney still out with suspension and Radamel Falcao a major doubt for the game after taking a knock in training. Angel di Maria is also suffering from a thigh strain but is expected to shrug off the issue and be back in first team action.

However, it will be a time for redemption for Juan Mata, who was unceremoniously sold to United by Mourinho last season as the Spaniard prepares to take on the Blues at Old Trafford.

"I had two fantastic seasons at Chelsea, winning titles and being voted the player of the year by both my teammates and the fans ... But he [Mourinho] came and I started to have limited time to play," Mata said.

"We had no relationship, there was no dialogue [between us]. I was not happy, but I always gave the most of myself and try to respect my colleagues. Then United came to sign me. I wanted to play the World Cup and I did not think much," he added.