The Spain international has been back in training since Friday, but assistant coach Jordi Roura had previously declined to confirm whether he would be available.
The 33-year-old took part in the Catalan side's last training session on Tuesday morning before the match, and should take his place in the starting XI as they look to overturn the two-goal deficit from the first leg.
"Xavi Hernandez has been passed fit to play in tonight’s big game against Milan after this lunchtime’s training session on the Camp Nou pitch," a statement on the official club website read.
|6/5||Barcelona are 6/5 with Paddy Power to qualify for the next round of the Champions League|
Goals from Kevin-Prince Boateng and Sulley Muntari gave the Italian side a 2-0 win in San Siro, and previous tournament favourites Barcelona have it all to do to reach the quarter-finals.