Mauricio Pochettino has told Nicolas Jackson to be smarter and stop picking up yellow cards for complaining to referees, with the Chelsea striker already only one booking from a one-match ban. Although the former Villarreal forward has shown plenty of promise since joining Chelsea for £32m in June, he has struggled to keep his emotions in check at times and has been booked four times in six appearances. Each caution has been for talking back to the officials and Jackson, who has scored once, has been warned about his behaviour by Pochettino. It is a worry for Chelsea’s head coach, who also spoke to Enzo Fernández about improving his discipline, because he lacks depth in attack. Armando Broja is working his way back from a long-term knee injury and Pochettino cannot afford to lose Jackson to suspension. “We were talking about that today because we were in a meeting with him and also Enzo,” he said. “Come on. A striker, four yellow cards only for talking with the referee? You need to get some yellow cards in different actions, not for that. “It’s so easy, cheap yellow cards, and I think it is going to put the team in a very difficult situation, but he understood. He is young, needs to learn, improve, settle himself. I think he is going to be a fantastic player but he needs time. He needs to be clever not to protest to the referees.” Chelsea, 14th before hosting Aston Villa on Sunday, remain short of ruthlessness. They have scored five league goals and will consider signing a striker in January. They are monitoring Brentford’s Ivan Toney. Pochettino said he had no regrets about not signing an experienced No 9 during the summer. His main worry at the moment is Chelsea’s massive injury list. They could be without 11 players against Villa. Yet Pochettino remains positive about his team’s displays. There is a spiritual dimension to his beliefs. Pochettino keeps a bowl of lemons in his office to absorb negative energy. He was asked when Chelsea will start to feel the benefits. “They started to work after two years at Tottenham,” Pochettino said. “Give time to the lemons. It is a thing that we all believe. If you want to have good energy you need to implement all the things that you believe. “I believe in the lemons but at Tottenham they started to work after one and a half, two years. They need a long time, they are not magic, but more than ever I still believe in them. “Today in my office I have yellow ones, green ones, different types. From Spain, from Italy. I don’t want to lie, there is a big box of lemons. I always thought the yellow lemons worked much better than the green but now I believe in any colour – any colour can help. If I could get a blue lemon it would be even better.” A debate broke out over whether a green lemon is actually a lime. Pochettino said: “A lime is not a lemon. It is a brother, maybe with a different mother or different father.”