The Costa Rican, signed from Eredivisie outfit Twente in 2011, was stretchered off against West Brom, while the Cottagers' boss was unimpressed with fan reaction to the result
The Costa Rica international was carried off the pitch towards the end of the game and the west Londoners are now sweating on his fitness, while a hamstring problem for Dimitar Berbatov is deemed less serious.
"Ruiz's [injury] looks quite serious so we have to scan him and wait and see what the problem is," said Jol. "It is with his ankle. I have no idea of the severity of it but we will see."
"Berbatov had a hamstring [problem]. I don't think that is serious. He gave us signal that he wanted to be substituted and hopefully it was just in time."
Fulham were booed off the pitch at Craven Cottage on Saturday, with audible chants of 'Jol out' after the Cottagers forfeited the lead late on following a host of spurned opportunities to double their advantage.
But the Dutch manager, who is unconcerned with his side's form, has hit back at fans who have called for his sacking and believes supporters have unrealistic expectations.
"At that end, there’s always five, 10 or 15 who shout at me," Jol continued. "It looked a bit more this time, maybe 25 or 30. I’m a very proud person and I’m used to playing in the top five or even higher.
"I came here to try and help the club and I would like to help them, make the fans happy. Hopefully I can do that and if not then we will have a problem.
"When people start yelling at you – I would like people to be a bit more appreciative. I can’t tell them to come outside the stadium after the game so I put up with it.
"I remember last year, I think it was the same geezer and I did something but luckily enough nobody saw it! It’s hard to fight expectation levels and maybe a minority feel we should be in the top four.
"We are 12th in the league and that’s maybe where I feel we could finish, maybe ninth or 10th. I think we are doing OK.
"Realistically, we should think the same as a lot of other teams – stay in the league then hopefully once all the players know each other, gel together then we should be in the top 10. I think I have managed to collect together a strong group of players.
"But if you play in a European cup final then the expectation levels are too high. That was three years ago. I played two semi-finals of the European Cup and I’ve forgotten about it."