You need to get the balance right when building a squad and Neil Lennon and Johan Mjallby want some old heads around. Celtic are looking to add some "experienced winners" to help bring trophies to Parkhead. Assistant manager Johan Mjallby says that is why the likes of former Middlesbrough midfielder George Boateng is being looked over alongside a number of other triallists who have joined the squad for pre-season training. Last week, Neil Lennon brought in South Korea international full-back Cha Du-Ri and Charlie Mulgrew from Aberdeen and has been working on bringing former England defender Sol Campbell and Hull midfielder Jimmy Bullard to the club. Mjalby considers mental strength to be one of the main requirements as Celtic's new management team continue their recruitment drive. Mjallby said: "It's important to have experience in the side. We like winners at this club. Sol Campbell, he has had a great career. George Boateng has done very well, he has been down in England playing for a few clubs there and played very well. It's always nice to get a couple of experienced winners in, guys who are very strong in their minds and know what they want on the pitch. That's what we are looking for. Even Jose Mourinho is probably looking for players in every department and so are we. When I first came here, we had a lot of good ball players in the side. We lacked a wee bit of steel. I think we lacked a wee bit of mental strength. You could see away from home we dropped too many points last season and conceded too many goals. If you want to be successful, you need to be able to grind out results, especially away from home. Be strong, win the battle before you can start to play good football. You want to win every game and you want to win in style, but you don't always get that chance." Other trialists at Lennoxtown are 22-year-old Montenegro forward Slaven Stjepanovic and 25-year-old Le Mans full-back Ibrahima Camara who are looking to earn a move to Glasgow. Mjallby added: "There are a couple of players trying to come in and being on trial. It's a good chance for us to have a look at them in training. George Boateng has only been here a day and there has been a lot of running. Hopefully he can stay on for a couple of days and we can have a good look at him when it comes to football as well. Slaven has been training for a few days with us. For all the players the first few days has been just running and not too much football. We have started to get the balls out now and it's a better chance for us to have a look at them." Celtic also remain hopeful of signing winger Joe Ledley from Cardiff. The 23-year-old has been shown round the club's training facilities and Celtic have an advantage over English clubs in the race for his signature given he would be able to move for free under cross-border transfer rules. Mjallby said: "I hope Joe Ledley thinks the same, that it's an advantage for him to play for Celtic. It's a great chance for him to win trophies and have an even more successful career. If you have a chance to play in Europe and play against the best teams it's good for your career and a chance for you to develop even more."