"So he went to see a specialist and he recommended the surgery being done, and it was done (on Tuesday).

"We think it has been a success but we are looking at quite a considerable time out still.

"Best-case scenario is probably six to eight weeks but it may be longer than that. We will see how he settles after the surgery."

Lennon, meanwhile, is not paying too much attention to Sparta's selection problems ahead of their game in Glasgow.

Sparta's squad has been decimated by health issues and injuries, with first-choice goalkeeper Milan Heca and Slovakian stopper Dominik Holec both out.

The Czech club will also be without teenager Adam Hlozek, Libor Kozak, Filip Soucek, Matej Hanousek and Bulgaria's Martin Minchev although it is not clear yet what part – if any – is being playd by coronavirus issues.

The Hoops lost 3-1 to AC Milan at home in their Group H opener before a 2-2 draw with Lille in France last Thursday, while Vaclav Kotal's side are bottom with no points after two defeats.

"I am just concentrating on our outlook on the group," Lennon said.

"We know that if we can get a positive result on Thursday night, it puts us in a decent position again. Obviously Milan and Lille are playing each other.

"I have sympathy for Sparta, like I say. It is not an easy thing to deal with as a coach and sometimes these things are out of your control, but in terms of ourselves we are looking forward to the game after a great win (over Aberdeen) at the weekend."

"We are not taking the game for granted, we will be very strong going into the game and we are looking for a positive result no matter who is involved with the opposition.

"I have sympathy for Sparta, like I say. It is not an easy thing to deal with as a coach and sometimes these things are out of your control, but in terms of ourselves we are looking forward to the game after a great win (over Aberdeen) at the weekend."

Lennon was pleased to see striker Leigh Griffiths back in the Scotland squad for the Euro 2020 play-off final with Serbia and Nations League games against Slovakia and Israel later this month.

The Northern Irishman was unhappy when the 30-year-old forward returned to pre-season training out of condition and, although he is not regularly starting games, goals against St Johnstone and Aberdeen off the bench indicates a return to form.

Lennon said of Griffiths, who could make his first Scotland appearance in over two years: "I am delighted for him. I think playing for Scotland means a lot to him.

"It is just another positive step in the right direction. He is working hard and continuing to work hard and obviously he will be in contention (on Thursday).

"He is in reasonable condition now and he has come back looking as hungry as ever.

"I understand him and I do know he has his ups and downs. We have shown a lot of patience with him and we are getting rewarded.

"It is down to him, he needs to stay at that level but he will be an asset to us as we go along."