The winger suffered a deep gash to the back of his ankle – so deep it exposed the achilles tendon – after Reds defender Jon Flanagan tackled him from behind in the 4-1 defeat at Anfield on Saturday.
Downing played on but was substituted and sent to hospital to receive stitches after he had it examined at half-time.
Manager Sam Allardyce was left fuming after skipper Kevin Nolan was given a straight red card for an identical tackle on Jordan Henderson late in the second-half, while Flanagan’s challenge on Downing wasn’t even deemed a foul by referee Michael Oliver.
Allardyce said after the game: “I wouldn’t disagree with Nolan’s red card, but I would say that there was a very similar if not worse challenge from Flanagan in the first-half on Stewart Downing.
“Henderson has not ended up in hospital but Stewart has and he needs 16 or 20 stitches in a massive gash across his calf and Achilles tendon.
“There is more danger in Flanagan’s challenge than there was in Kevin’s but you couldn’t argue that he sent him off. What you do argue is that Kevin has been sent off and he [Flanagan] hasn’t.”