The former Kilmarnock and Dundee forward signed for Saints in the summer but has now joined up with old Rugby Park team-mate Lee Johnson after agreeing a six-month loan switch to Oldham.
Harkins could make his debut for the Latics at Anfield when Liverpool host the Sky Bet League One outfit in Sunday's FA Cup third round tie.
But after handing him just nine starts for the Paisley side, Lennon admitted the player's lack of work rate was a worry.
He said: "Things are never guaranteed to work out but Gary was a top bloke and I didn't get one bit of bother from him.
"He has fantastic ability and I can understand why he wants to go out on loan to play. But the way we are shaped up at this moment we can't guarantee that.
"He can create things and open doors but we need guys who will be compact and put everything in. I would have liked to have seen him work a little bit harder out of possession.
"I thought we he came here we could have changed that ever so slightly but it hasn't worked out as well as we had hoped for."
St Mirren host Celtic on Sunday hoping to become the first club to defeat the runaway Scottish Premiership leaders.
They will pin their hopes on new signing Adam Campbell, who took just 17 minutes to open his account after switching on a loan deal from Newcastle when he struck the opener in Thursday's 2-1 loss to Kilmarnock.
And the 18-year-old hopes to make a similar impact to Paul Dummett, who moved north from St James' Park last year alongside current loanee Conor Newton before helping the Buddies lift the League Cup. He has now returned to England's north east to cement himself a slot with Alan Pardew's first team.
"I saw Paul and Conor come up here last year and do really well," said the England Under-19 international. "As soon as the manager told me St Mirren were interested I jumped at the chance to come here.
"I would love to do what Paul has done. He came here, won the cup and has gone back to Newcastle, got a new deal, scored against Liverpool and played in the derby. I want to do all that myself.
"I got off to a great start with a goal 17 minutes in. You always want to score as early as you can so it was great to get that burden off my shoulders.
"I don't set myself targets, I just want to get as many as I can. Hopefully I can make double figures but I'm not putting a number on it.
"We've got Celtic to come on Sunday against Celtic. They are the biggest club in the country and have down well in Europe recently, putting up a decent show against Barcelona.
"It will be a big test for us and I'm looking forward to it."