Johnston smashed Azhar Mahmood over long-on for six to seal a famous three-wicket victory to put the 1992 champions out of the competition, but he had injured his shoulder while fielding earlier in the day.
"I've got some sort of rotator cuff problem in the left shoulder, after I injured it diving for a ball," said Johnston. "It's an old injury. There's quite a bit of pain there - I'm going to hospital to get an MRI scan.
"Hopefully, I can get it right in the next six days, so I've got to take it easy, and watch the rest of the boys party."
Ireland face hosts West Indies in the final group match on Friday, and he is anxious to be fit for that contest, after his side made St. Patrick's Day one to remember for the scores of Irish supporters in the stands.
"I didn't do too well in English at school, so I don't have a word for it," Johnston said.
"It's amazing. We had a huge advantage in winning the toss. We bowled the ball in very good areas, and we were lucky they snicked off, and we held our catches, and we had a total of 133 to chase which was unbelieveable.
"It's magnificent. The support out there also was amazing for us. That was another thing which carried us to the victory. It was those 1,500 Irish supporters up there in the stands."
Johnston felt his teammates deserved a night to indulge in themselves in a celebratory drink, after the way they played.
"I just think we kept plugging away, and I never thought Pakistan gave up," he said.
"With the quality of their bowling attack, and the experience they have in one-day international cricket, 130 on that pitch is like 270 on a flat pitch, and they always had the bowlers to bowl the balls in good areas and get us out.
"We are not used to that sort of pressure, but they are accustomed to it day in, day out, but our guys just stood up, and I can't say enough about the two O'Brien brothers, and how they batted - it was outstanding. Their guts and courage got us over the line."
Niall O'Brien hit a one-day international career best 72 from 107 balls to be the backbone of the Irish chase.
He shared valuable partnerships of 47 with William Porterfield for the third wicket, and 38 with his brother Kevin for the fifth wicket to consolidate for Ireland.
"These two games in the World Cup have helped us to put our experiences at the World Cricket League in Kenya to bed because we lost four close games there," Johnston said.
"To come back against Zimbabwe (a tied match last week here), when they should have beaten us was a fantastic effort, but the way we played in this match was phenomenal.
"We wanted to bring our A-game to the party and we did that. The fielding was superb. We held every catch.
"The batting was awesome against a first-class attack on a pitch that was doing stuff. It was a great victory."
Johnston felt his side's victory would give the game a boost in Ireland.
"We came here to show people that there is good cricket being played in Ireland and some of the ICC Associate nations," he said.
"We are just going there to put in a good old fight. I'm sure it's going to life the profile of the game in the country."
The win over Pakistan put Ireland in the lead of Group D with three points - one more than West Indies and two ahead of Zimbabwe (both with a match in hand) - leaving Pakistan at the bottom of the table with no points.
It also means they are on the verge of qualifying for the Super Eights.