The Senegalese striker, a key figure for West Ham in the last two seasons, has just returned to fitness after being out since the start of the season with a back injury and scored early in Sunday's draw before going off in the second half.
Manager Slaven Bilic said he would miss the return visit to Old Trafford and fears he faces another spell out.
"We will have to wait for the results of the scan to see how many weeks we will be without him," Bilic told a news conference.
"It is a big blow because we know what he can give to us and he showed that against Manchester United on Sunday. I can only hope it is not going to be long.
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"To be sidelined again is awful for him but I am sure this injury will not be like the one that forced him to miss the start of the season."
Better news for West Ham is that injury-plagued striker Andy Carroll could be in line for a return having been out since August with a knee injury.
"Andy has had a good recovery and has been training with us," Bilic said. "If he progresses well and there are not any minor setbacks he will definitely be ready for Saturday and we will make a decision about the United game after training."
West Ham travel to United just two wins away from a first domestic cup final since losing to Liverpool in the 2006 FA Cup final.
However, they are only one point above the Premier League relegation zone and host Arsenal on Saturday.
"The league is always the priority but this is a big game and the cup competitions are important for us," said Bilic who hinted at some team changes.
"It will be a big thing to reach the semi-finals and that is why we are approaching the game in a positive way. It would give us a lot of confidence if we could win the game."
The teams drew 1-1 at the weekend.
(Reporting by Martyn Herman; Editing by Tony Jimenez)
