Sigthorsson returns after suffering the injury while playing for Iceland in the first leg of last month's World Cup playoff loss to Croatia.
But Ajax will be without injured captain Siem de Jong, Nicolai Boilesen and Lerin Duarte, while defender Joel Veltman will miss the match due to suspension, the Dutch champions said on their website (www.ajax.nl).
De Jong has a hamstring injury, picked up on international duty with Netherlands last month, while Duarte has ankle problems.
Boilsen injured his hamstring during Ajax's last Champions League game, a 2-1 home win over Barcelona that kept the Dutch champions in contention for the knock-out phase.
Ajax must win their final Group H match at the San Siro to advance to the next stage.
Barcelona, top of the group with 10 points, are already through, but Milan, second on eight points, and Ajax, on seven, will fight it out for the second qualifying spot.
Celtic have been eliminated with three points from five matches.