Eastman sidelined with ankle injury
It is a big blow for the former Ipswich Town youngster, who has been a regular for the U's over the past few seasons.
An operation looks likely and the suggested recovery time is at least three months.
At the end of the Blackpool game, the centre half landed awkwardly and had to limp off, and he will now be out of action for a while.
U's defender Tom Eastman is likely to miss the rest of the season.
Over 200 appearances have come since his move from south Suffolk in the summer of 2011, at an average of approx 40 games a season.