The Scotland international replaced Nir Biton at half-time of the weekend's clash with Heart of Midlothian to spark the Hoops into scoring two goals in the second half, the second by him, in the 2-0 win at Celtic Park.
Forrest was the difference in Mjallby's opinion and the Swede made it clear just how important a player the 22-year old is to the Hoops' line-up.
"When James Forrest came on for the second half, he gave us the spark and I think we controlled the game," Mjallby 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."
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