With the first-half goalless and the league leaders struggling to break down a resolute Jambos side still smarting from the 7-0 Scottish Cup thrashing when the two sides met at Tynecastle at the start of the month, the Scotland winger, back from a hamstring complaint, replaced midfielder Nir Biton at the interval.
Top scorer Kris Commons eventually got the breakthrough in the 63rd minute with his 13th goal of the season before clearing a Dylan McGowan header off the line in the dying moments.
And Forrest made sure of the points in injury-time with a well-taken goal on the break as Neil Lennon's side extended their unbeaten league run to 16 games this season.
Mjallby is hoping the former Celtic youth player can put his injury troubles behind him for the rest of the season.
"When James Forrest came on for the second-half he gave us the spark and I think we controlled the game," he said.
"He was always a threat for us.
"A fit James Forrest will always start every game but it is hard keeping him fit this season.
"We need to try to keep him fit and have him more involved in the team because he is a special player for us."
Both players' contracts are up at the end of the season with manager Neil Lennon admitting at the pre-match press conference on Friday that he is bracing himself for their departure.
Mjallby, however, is confident that attitude will not be a problem, regardless of what respective decisions they make.
"There is no concern whatsoever," said the Swede.
"They are two great professionals and I think Joe Ledley has been playing very well recently.
"The way I understand it is that there are offers on the table and the sooner we get an answer from them the better because we need to look further ahead, we can't just look forward the next couple of weeks.
"If they are not likely to be here then we would like to get some money for them and would probably look for replacements in those areas."
It was a valiant performance by the administration-hit Jambos who were keen not to suffer again at the hands of the Hoops but ultimately it was another defeat.
Gary Locke's young squad was hit by injuries which meant he could not even fill the bench.
Only five substitutes were named as the former Hearts skipper refused to fill the quota for appearance sake.
"I brought a couple of young lads with the squad but they had just left school in July, so I didn't think it was right for me (to put them on the bench)," he said.
"You have to come into the first-team squad when you are ready.
"We only had enough for four outfield players on the bench but there is nothing we can do about that, that's where we are at the minute."