French referee Pierre Brousset did not feel a red card was justified at the time of the incident, which occurred in the 69th minute of the match, but citing commissioner John Montgomery has since intervened.
That meant Wood had a hearing online on Wednesday, and he was subsequently suspended.
He will be free to return to action from Monday January 11, meaning he misses Saints' key Premiership matches against Worcester Warriors, London Irish and Leicester Tigers.
An EPCR statement read: "An independent Disciplinary Committee comprising Roger Morris (Wales), Chair, Marcello d’Orey (Portugal) and Frank Hadden (Scotland), considered video imagery of the incident and heard submissions from Wood, who accepted that he had committed an act of foul play that warranted a red card, from the player’s legal representative, John Shea, from the Northampton Saints Team Manager, Maurice Hartery, and from the EPCR Disciplinary Officer, Liam McTiernan.
"The committee upheld the citing complaint, finding that Wood had made contact with van der Flier’s neck and head area in a dangerous manner that warranted a red card. It then determined that the offence was at the mid-range of World Rugby’s sanctions and four weeks was selected as the appropriate entry point.
"There were no aggravating factors and taking into account the player’s guilty plea and expression of remorse, the committee reduced the sanction by one week before imposing a three-week suspension."