Ismael Bouzid was thrown straight into the starting line-up for the Scottish Premiership clash at Firhill after completing his move to the Rugby Park side on Friday.
And former Killie winger David Silva, who was also offered a deal after impressing on trial, is set to complete his return to the club in the coming days.
Johnston said: "He's agreed everything. He should be coming in this week and hopefully he will add a bit of pace and creativity in the wider areas."
On Bouzid's display, he added: "That's what we need, a big physical presence at the back. We have been lacking that all season.
"You can see that he's a leader and an organiser at the back.
"Hopefully he will improve us as a team. I thought he had a good solid performance for his first game."
An Aaron Muirhead penalty gave Partick a well-deserved lead at the interval, before Kris Boyd netted early in the second half to earn Killie a share of the points, as their search for their first win of the season remains ongoing.
Johnston said: "It was a poor first-half performance. Thistle completely controlled the first half.
"We are not starting games as well as we should and we are letting teams get into a lead and putting ourselves under pressure.
"We've got good players and I felt for the majority of the second half we were the better team.
"We were unlucky not to go on and score more goals and win the game.
"They put us under a bit of pressure in the last 10 minutes which, as the home team, you would expect them to do.
"But, overall in the second half, it was a much better performance."
Thistle boss Alan Archibald also felt his side should have won the game with an improved performance on last week's home defeat to Aberdeen.
He said: "I was delighted with the way we bounced back from last week.
"The players took everything on board and we started the first half really well.
"I think if we'd got another goal, we would have put the game to bed. At 2-0, it's a different story.
"The goal gave them a lift and we had a shaky 15-minute spell and Kilmarnock had their best spell of the game.
"But, after that, I felt we had the better finish to the game and I thought we were going to get the winner."