Celtic must quickly step up their transfer business this summer or risk falling behind Europe鈥檚 elite next season, according to a former Parkhead hero. Head coach Brendan Rodgers has secured the free transfer signings of left-back Kierney Tierney and goalkeeper Ross Doohan, while a 拢1.2million deal for Swedish international Benjamin Nygren is also reported to be close. However, the Celtic boss has been urged to take his transfer business to the next level and begin to make real progress in the summer market by ex-Hoops forward Charlie Nicholas, who admits he is concerned at the lack of movement at Celtic Park in the opening weeks of pre-season. 鈥淐eltic need to get moving in the transfer market,鈥 said Nicholas. 鈥淭he Scottish clubs come back for pre-season a couple of weeks before their English counterparts, due to the European qualifiers. They鈥檝e already started this week, but they鈥檒l need to see who goes out the door before they know what kind of budget they鈥檇 have. 鈥淏rendan Rodgers has been through this every summer in his two stints at Celtic, and they鈥檙e needing a bit of a rebuild 鈥 especially with Rangers getting new ownership and are more likely to pose a real threat to Celtic this season. Celtic in Europe had a wonderful draw last season, and I鈥檓 not as sure they鈥檒l get as luck this term if they get through.鈥 Nicholas also revealed the number of new signing he feels the Hoops will require if they are to make an impact on Europe鈥檚 big stage next season, and admits there is genuine concern that they could lose some of their best players before the new season starts. 鈥淚 think they need another five players,鈥 said Nicholas. 鈥滳ertainly another centre-forward – if not two. Daizen Maeda might want out, and Reo Hatete might as well. There鈥檚 a lack of action. They need to start getting people in. We鈥檝e got Callum Osmand joining from Fulham who is going to be a backup, but there鈥檚 no sign of any positive movement in the transfer market. I reckon they鈥檒l be six or seven players going out the door, and they鈥檒l need to sign at least five to compete in the Champions League. Despite his concerns over Celtic transfer business, the ex-Scotland international believes his former side has little to worry about domestically as he discussed the appointment of Russell Martin as Rangers鈥 new head coach. 鈥淚t鈥檚 too early to say whether Russell Martin is the right manager for Rangers,鈥 Nicholas said. 鈥淗is philosophy at Southampton in the Championship was impressive, and his team had a high press, and played with energy and pace. Rangers have become a lazy side. They want to get and keep the ball, but don鈥檛 want the other side of it. Only a few players, like Nicholas Raskin and Mohamed Diamonde, want to do that work, and the other players don鈥檛 blend in. Martin will bring in a group and a determination. 鈥淭he players are surely looking season after season at having another barren campaign, and it鈥檚 becoming a source of embarrassment. They鈥檝e not even come close to the title. If they start beating the domestic teams, then they can probably get more results against Celtic. It鈥檚 going to be much tighter, and he鈥檒l get more players in and give them a better shape. 鈥淚f he makes them more united as a group, they鈥檒l be contenders. He鈥檚 got to slowly build this squad, and when the time comes to hit the ground running domestically, they must be ready to go. It鈥檒l be much tighter, but they still have a lot of catching up to do with Celtic.鈥 Charlie Nicholas was speaking to Sky Bet. Sign up to The Scotsman鈥檚 daily football newsletter to get unrivalled Scottish football news and analysis – subscribe for free here.