The pair missed Spain's recent internationals but Mourinho says striker Costa has "fully recovered" from hamstring and groin complaints, while Fabregas is in contention after returning to training on Friday after a minor thigh tear.
John Obi Mikel will also be assessed having returned from Nigeria duty on Friday, while Ramires is available for selection albeit "not in very, very good condition"
Nathan Ake is out for a few weeks with a hamstring injury but Andre Schurrle is back.
A fully-fit Costa is a worrying prospect for Chelsea's Premier League rivals and, in particular, West Brom.
It is a match the west Londoners head into top of the table with 29 points from the first 11 matches - a start only bettered twice in Premier League history.
Such fine form had even led one bookmaker to pay out on the Blues winning the title, although Mourinho scoffed at the suggestion that the title race was all but over.
"No complacency, that's for sure," Mourinho said
"If we lose matches, and I believe we will, it won't be because of complacency
It will be because football is football.
"It was not in the Premier League but I can give you an example: our game against Maribor
We lost two points, but it wasn't down to complacency
It was because of football.
"Their goalkeeper was the man of the match and we missed so many chances
My team played tremendously well in the second half but we still lost two points.
"You lose points because it's football, because the opponents are better than you, but not because of complacency."
Craig Gardner could return to the West Brom line-up after suspension
Boss Alan Irvine's only injury concerns are defender Jonas Olsson, still out with an Achilles problem, and left-back Sebastien Pocognoli, who has a hamstring injury.
Former Everton assistant-manager Irvine has demanded his side attack Chelsea instead of 'parking the bus'.
"We can't be passive," he said
"If we sit back and let them have the ball and try to soak up pressure for 90 minutes then eventually they will open us up.
"If you sit back and don't take the opportunities to put them under pressure with and without the ball eventually they will probably win."
Source : PA