Johnson, who retired from playing at the end of last season and has been working as the club's youth-team coach, is in charge of first-team affairs along with assistant manager Dave Kevan following McParland's sacking this week.
McParland lost his job with the Magpies fifth in League Two and just four points off the top of the table. They were fighting for their Football League survival when the Scot arrived at Meadow Lane in October 2007.
Johnson, who was signed by McParland, said: "Charlie (McParland) has left the club in a much better position than when he came. The club is miles better off because of him and the work he did. When he took over, the club was in a bad way and he saved it.
"He didn't just put together a good first-team squad, there is now a youth set-up and reserve team. He's done an immense job and whoever comes in only has to put the finishing touches."
On McParland's dismissal, Johnson added: "I'm shocked by the timing of it. It's not like it's on the back of three or four defeats. The expectation is so high now and there have been rumours almost every day about other people coming in to take over."