The Belgium international had been struggling with a calf problem but the 27-year-old has been able to shrug off the injury. He is expected to start Wednesday night's encounter with the Gunners at Stamford Bridge.
New signing Ross Barkley is not available for the London derby as he is still recovering from hamstring surgery.
Chelsea boss Antonio Conte has confirmed he will be making changes to his squad for the first leg although the Italian is taking the competition seriously and is determined to make the final at Wembley.
He said: "When you play every three days, it's important to make rotations, but at the same time your team must be very strong, otherwise you risk dropping in this trophy.
"It's a very important game for us, a semi-final and the chance to win a trophy. We deserve to be here and it will be a tough game against Arsenal."