Neil Lennon, the Celtic manager, last night warned Derk Boerrigter and Amido Balde they will have to "improve dramatically" to have a future at the club next season.

Boerrigter, Balde and striker Teemu Pukki have all had underwhelming debut campaigns with the champions, who will lift the SPFL Premiership trophy after tomorrow afternoon's final game against Dundee United. But while Lennon was prepared to exempt Finnish striker Pukki from scrutiny, he admitted Dutchman Boerrigter and former Portugal Under-20 striker Balde had not lived up to expectations.

Boerrigter, 27, arrived with a Champions League pedigree from Ajax and cost around £1m. Balde, 22, cost £1.8m from Vitoria Guimaraes and it took £2.4m to land Pukki, 24, from Schalke. None of them has provided value for money so far. Pukki has scored seven times in 16 starts and 15 substitute appearances. Boerrigter, often injured, has only one goal from eight starts and 14 games from the bench. Balde has managed three goals from three starts and 20 substitute appearances.

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All were signed last summer, as was the excellent Virgil van Dijk, and when asked to describe their first campaigns Lennon said "disappointing". He went on: "I think Teemu has got better as the season's gone on. Amido will probably say that he hadn't had enough game time. Derk has been littered with injuries. They will maybe think that they haven't had the opportunity but they must do better. I was really pleased with Pukki on Wednesday night [he scored in the 3-3 draw at St Johnstone]. I think he still has got something to give. The other two need to improve dramatically if they're going to make any sort of headway in the first team for next season.

"With Derk it's more frustrating for him than anyone else because of his injury situation. It has been a bit like James Forrest, although Jamesie has played more games. Derk gets in for a week and then is out. He has even had a couple of injuries in training that have kept him out. I think he will be more frustrated than anyone else with the way it's gone."

Celtic could consider letting Balde leave on loan, just as Tony Watt nears his return after a season-long loan to Lierse. "We did have interest in Amido in January but we didn't want him to go. Whether he wants to go somewhere and play consistent football, I don't know. We'll sit down and talk about it, maybe in pre-season. But for the time being they are still part of what we are doing for next year.

"It's the same as any other player, I wouldn't just single out any of those three. I want to improve the team and the squad and we will listen to offers for some players. We won't encourage bids, but if bids come in we will look at them. If it's good business for the club then we'll consider it.

"Some buys work for you and some don't. You can't judge them on one season. Maybe they need a second season to really show what they are capable of. The majority of British players we have brought in have worked, but they don't always work. Again, it's an important time between now and the [Champions League] qualifiers to try to find the right ones to make us better."