Many Kilmarnock fans were disappointed to see their top goalscorer for the past two seasons leave on the eve of their 2-1 Scottish Premiership defeat by Motherwell at Fir Park.
But Johnston hinted that the Irishman's agent had been seeking a new club for the forward for several months.
Johnston, whose team failed to build on a 17th-minute lead at Fir Park, said: "Heff's a good lad but he wasn't too happy playing up front with Boydy (Kris Boyd) as well and I think he wanted to leave.
"If he wants to leave then it's better for us if he moves. It frees up a lot of money because he was probably the top-paid player at the club.
"So we should hopefully be able to bring in a couple of players.
"He never asked to leave but his agent said Hibs and Motherwell were interested in him at the start of the season and that just came about again."
Johnston handed Michael Gardyne a debut off the bench following his loan move from Dundee United and watched his side push Motherwell all the way.
Killie only held the lead for three minutes when Henri Anier levelled after Stephen McManus had put through his own net, and a fresh-air swipe from Jackson Irvine allowed John Sutton to net on the stroke of half-time.
Boyd came close on several occasions before Motherwell goalkeeper Lee Hollis rescued his team with two late saves following Well right-back Simon Ramsden's red card.
Johnston said: "You can't afford to defend the way we are defending and win games. We are making it an uphill battle week in and week out.
"I thought we played really well. I thought we dominated, especially the second half, passed the ball really well and looked really confident, and you can see we are not far away."
Johnston will try to bring in two free agents after the transfer window shuts.
He said: "Hopefully with the money we have saved with Heff moving on, we should be able to strengthen.
"We have been trying to bring in a centre-half and a couple of them fell through that we tried to bring in on loan.
"There are a lot of players available and I still think we need to bring in another two."