St Mirren's Kenny McLean to undergo knee surgery
St Mirren midfielder Kenny McLean will be out of action for up to six weeks, with the 21-year-old poised to undergo a knee operation on Friday.
McLean, who had already missed the Buddies' last two games, was left out of the squad for
"That's a bit of a blow for us, certainly in the fine form he was on," Lennon told BBC Scotland.
"You are probably looking at four to six weeks."
The Buddies held out for almost an hour before Celtic fired in four goals in a blistering 15-minute period that dismantled the Paisley side in Sunday's encounter.
Lennon said the first goal against Celtic often decides a match and it can be difficult to recover, despite a positive early period in which Celtic struck the same post twice in their search for an opener.
"We came in and I thought we were more than a match and worthy of a 0-0 at half-time," said Lennon.
"Although, against a quality side like Celtic, you need a bit of luck. We got that with them hitting the woodwork twice.
"We were growing in confidence and I thought we were looking dangerous on the break. We got good balls into decent areas, but they're a difficult side to break down.
"I think when you concede the first goal against Celtic, you talk about how you're going to cope with it - and it affected us very poorly.
"We gave the ball away needlessly and when you do that against a quality side like Celtic, they punish you."
Lennon explained before Sunday's kick-off that Scotland Under-21 international McLean had been nursing the injury for several weeks but had failed a fitness test ahead of Thursday's defeat by Kilmarnock.
"The little episodes he's had with his knee, normally it settles down within three to four days," said the St Mirren manager.
"But he's feeling it and he's got to go under the knife, so I think it's next Friday that he does so."
Striker Adam Campbell has
However, the and the for Swansea City goalkeeper David Cornell, Blackpool midfielder Jake Caprice and Nice striker Stephane Bahoken gives Lennon some room for manoeuvre in the transfer market.
"The squad is really stretched at this moment in time, but the January window certainly does give an opportunity to try to bring in one or two to help the group along," he said.
"We are very active on that front at the moment. It is inappropriate for me to mention any names, because you're never guaranteed anything at this stage of the season, but all I'll say is watch this space."
Meanwhile, Lennon revealed that injured winger Gary Teale is "making good progress and should be back for us by the end of the month".