Isak was introduced as a half-time substitute in north London and opened the scoring against 10-man Spurs with a cool finish in the 56th minute.
However, the Sweden international was immediately forced off after a coming together with Tottenham defender Micky van de Ven inside the penalty area in which he appeared to injure his right knee.
He did not need a stretcher but looked in a lot of pain as he hobbled off the pitch, with Liverpool now facing an anxious wait to discover the full extent of his injury.
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It is yet more disappointment for the 26-year-old, who has endured a difficult start to life on Merseyside following his acrimonious summer move from Newcastle United.
The £125 million ($167m) striker has scored just three goals in all competitions since joining Liverpool on transfer deadline day and was sidelined for a spell earlier in the season with a groin problem.
Hugo Ekitike scored a second goal for Liverpool in the 66th minute at Tottenham Hotspur Stadium before Spurs threatened to claim an improbable draw when Richarlison pulled a goal back in the 83rd.
But the home side finished with nine men after defender Cristian Romero was sent off in stoppage time, allowing Liverpool to secure a valuable three points.
