Ruiz, 27, last played in Fulham's 3-1 defeat to Sunderland three weeks ago and had been expected to return against Newcastle United on Monday.
"As you know we were very happy to tell everyone last week that Bryan Ruiz was back on Monday, and at the same training session he was injured again and we had to send him to Germany for a second opinion," Jol said on the club website (www.fulhamfc.com).
"He is out for four or five weeks with the same hamstring problem. Hopefully he will be back before the FA Cup match."
Fulham host Blackpool in the third round of the FA Cup on Jan. 5. (Reporting by Toby Davis; editing by Robert Woodward)