The French forward suffered an ankle sprain in the 1-0 first-leg defeat by Inter Milan in February and missed the following four matches across all competitions, in which Atletico won just once.
Griezmann has trained ahead of the second leg at the Metropolitano Stadium in the Spanish capital on Wednesday and could start.
"Griezmann gives the team a lot of strength... he's a very important player, we are very happy that he is back," Witsel told a news conference.
Coach Simeone said the 32-year-old's return would expand his options in the clash against last year's runners-up.
"His return to the team gives us more solutions and more tactical possibilities... and we hope he has the game that he wants to have," said Simeone.
Atletico were beaten 2-0 on Saturday by lowly Cadiz in La Liga but the coach expected supporters would back the team to the hilt against Inter.
"We'll accept what they want to give us - they always give everything and support us, I am sure our fans will be like they always are," added Simeone.
Atletico are fourth in La Liga and facing a battle with Athletic Bilbao to hold on to that spot, while the Basque team knocked them out of the Copa del Rey in the semi-finals.
"I can say that I believe in my players, in my team and I know they will have a good game," said Simeone.
"In recent games the team has not maintained the level they had been playing at and we hope to keep growing after the lessons (we've been given)."
