Diarra's form at Fulham has seen him return to the national team set-up, meaning he may well soon be heading to the African nations Cup.
"We'll look to strengthen the squad because if Diarra goes I can do nothing about it," Jol said. "As it is now he will go to the African Nations Cup.
"He was so good for me last year because a lot of older players left the club. He changed the dressing room.
"I can remember him when he was 18 he was a revelation. He started in Crete and then came to Holland with Vitesse.
"There won't be a lot of players in Europe who will stay at Real Madrid for five years so that is how good he is.
"It was a gamble because he was 30 so it's important to keep him fit."