Club doctor Ges Steinbergs, having discussed the results of the scan with the physiotherapy team, says: "It is something that could take two to three weeks to clear up, possibly more depending on how it settles down."
However, with the game at Merseyside two and a half weeks away, Ges could not definitely rule Fredi in - or out.
"Normally this sort of thing is two to four weeks, but Fredi has been a quick healer in the past," he explains.
"It is difficult to gauge at the early stages."
Ges tended to Fredi after he pulled up sharply in the 60th minute at Stevenage Borough, and reveals: "It all came out of the blue; I asked him if he felt any tightness before and he said: 'No, I felt really good.'
"Sometimes whan you are tired you get injuries, but he said he felt strong and it just happened."
If he does miss the opening game at Anfield, it will be a huge blow to Fredi, who has scored five goals in three friendlies and recently committed his future to West Ham.
Of the other injured players, Scott Minto (thigh), Titi Camara, Steve Lomas, Ian Pearce and Shaka Hislop (all with knee problems) are still in various stages of rehab.
Svetoslav Todorov is back in action at Braintree tonight having had his stitches out on Monday, while Joe Cole (bruised foot) should be back in action on Friday at Notts County.