CELTIC last night completed the £3m signing of Derk Boerrigter, the Dutch winger, on a four-year deal from Ajax.

"We're delighted we have got him," said Neil Lennon, the Celtic manager after the 26-year-old had undergone his medical. "He is the type of player we have been after for quite a while. He is quite pacy and predominantly left-sided. I would think the Champions League is a factor in him coming here. Off the back of what we achieved last season, we are maybe a bit more attractive than we were previously."

While Lennon was happy to talk of his new arrival last night, his mood earlier in the day had been decidedly different when he hit out at "naughty" behaviour by Nottingham Forest over their attempts to re-sign Kelvin Wilson, the Celtic defender.

Celtic rejected a £1.5m bid from the Championship club last week but Lennon believes there is an attempt to unsettle the centre-back who joined Celtic from Forest in July 2011 on a free transfer.

Lennon, speaking before tonight's Champions League qualifier against IF Elfsborg at Parkhead, said: "There is no doubt there is something naughty going on. It doesn't make me happy, because we don't do that type of thing. As soon as the bid was made on Friday, it was out on BBC East Midlands about 20 minutes later. That's not the way you do things."

Lennon is determined to make every effort to keep the defender. "The last couple of games Kelvin has played, he has been excellent. He is being as professional as he can. These are edgy situations, where a player might have his head turned," he said.

"We have to do all we can to convince him to stay. I'd like to think he would want to stay, but I can't say. His home and family are in Nottingham and that's a big lure for him. Some people down the road know that and are using it."

The Celtic manager described Wilson as "my No.1 centre half", adding: "Having lost Victor [Wanyama] and Gary [Hooper], I certainly don't want to lose any more players.

He also was dismissive about the size of the Forest bid. " Look, we don't put prices on players but £1.5m is well short of our valuation and what we think of him," he said.

Of Wilson, he said: " I want him here and I want him happy. I just need to do a bit of work on him. In the meantime he will be in the squad for the Elfsborg game. He has trained well and has been playing well, and he looks okay. But obviously there is something going on, people trying to ruffle his feathers a little bit."

He said of the reports emanating from south of the Border last night that a deal to take the centre-half to Forest is nearing completion: 'There is just nothing in it. We had a bid last Friday and knocked it back. There has been nothing else since then."

Lennon remains focused on signing a striker: "We've been knocked back on a couple of targets but we are working really hard on it," he said.