The Hoops had thrashed the administration-hit visitors 7-0 in their Scottish Cup clash at Tynecastle at the beginning of the month and Jambos boss Gary Locke was keen to ensure there would be no repeat.
However, Commons opened the scoring in the 63rd minute with his 13th of the season before Forrest, on at the start of the second half for Nir Biton, clinched the points with a second in injury-time.
Hoops assistant Mjallby believes the Celtic players and management need "new targets" after being knocked out of the Champions League - the first of which is to go another four league games unbeaten.
He said: "Obviously there is not going to be any more European football this season, so we have to set new targets and incentives for the players.
"Being unbeaten is going to be very important for us and obviously right now, the target is to go 20 games unbeaten and then we take it from there.
"You always look for progress and to do better. Being knocked out of Europe was disappointing but the first target was to get there and hopefully get the same run as last year (last-16 appearance) - but it is quite hard because you are fighting against teams who have bigger financial muscle."
Top scorer Commons also cleared a Dylan McGowan header off the line in the closing stages when it was still 1-0 for the home side.
However, he declared himself more pleased with the impact he is making at the other end this season.
He said: "Yeah, it is good. I am moving them along. I have not set any targets so it is not like I am going to be shooting from here, there and everywhere.
"I just try to stay in the starting 11 and hopefully keep scoring goals.
"It is nice when you are a bit of a vein of form. I have been on the other end of the stick where I have scored the one goal in the season so it is swings and roundabouts.
"For an attacking midfielder, anything in double figures is good. "I don't think there are many attacking midfielders that will be looking at 20-plus.
"That is the sort of thing you start looking for from your strikers.
"The last couple of seasons I have been here, we have always had two strikers who have got 20-plus, so me as back-up is good.
"It was never going to be another 7-0 because they made it very difficult for us to score goals but we did well to change things to add another dimension to our attacking play and James Forrest brought that into the second half."
While Celtic continue to move inexorably towards the title, a youthful Hearts side remain slumped at the bottom of the table and on course for relegation.
However, Locke, who had only five substitutes on the bench due to injuries, was glad that some pride had been restored after their cup thrashing by Celtic.
He said: "We learned a harsh lesson the last time we played Celtic but I thought to a man, we were fantastic.
"We kept our shape well and Dylan was a wee bit unlucky at the end or our game plan would have worked a treat. It was a far better performance.
"We tried to contain Celtic, the last time at Tynecastle we gave them far too much space on the ball and so we tried to be compact and I think we did that brilliantly.
"We asked the players to restore a bit of pride after the last game against Celtic and we certainly got that.
"It is difficult place to go to but we are not going to stay in the league by winning at Parkhead, it is the games coming up, they are big for us.
"Hopefully we will get one or two players back from injury and if they give us that commitment we won't be too far away."
Hearts goalkeeper Jamie MacDonald kept Celtic at bay with a string of great saves and had no chance with the goals.
He said: "I was just happy to make some saves this time. Obviously the last time we lost seven so from my personal point of view, I had a point to prove just as much as the team today.
"We all had to deal with that last Celtic game and learn from it - it was more of a feeling of embarrassment after that game.
"Unfortunately we didn't get a positive result today but I think we can take heart from the performance.
"We held out for 60 minutes but then didn't chase the game, we kept our shape and almost got an equaliser."