The Finland international striker opened his account for the season by netting 15 minutes into the 2nd Round tie at Glanford Park and he sealed the tie for Blues from the penalty spot 20 minutes from full-time.
Steve Bruce was forced into making a number of changes to the side that battled to a 1-1 draw at Portsmouth last Saturday.
Nicky Butt began his three-game ban for his sending off at Fratton Park whilst Saturday's goalscorer Jiri Jarosik failed to shake off an ankle injury. Damien Johnson was moved from a wide position to central midfield where he was partnered by Muzzy Izzet who was making his first start in ten months.
Jamie Clapham returned at left back whilst there was also a first start of the season for Martin Taylor, who replaced Kenny Cunningham, and goalkeeper Nico Vaesen.
Blues began brightly with Forssell hitting a post ten minutes into the game with a neat left foot shot on the turn.
Five minutes later and Blues took the lead courtesy of the same player.
Gray did well down the left flank and his cross was met by Forssell who slotted home from inside the box.
Neil Kilkenny was introduced for his Blues debut on 25 minutes when he replaced the injured Izzet.
For the remainder of the first half Blues played keep ball without really threatening The Iron goal whilst the hosts rarely threatened but for a free kick effort which was well saved by Vaesen.
Scunthorpe had the better of the early second half exchanges and found the net in the 62nd minute through Billy Sharp but the strikers effort was ruled out for off-side by the assistant referee.
Blues wrapped the game up in the 70th minute when a surging run into the box by Jamie Clapham was brought to an abrupt end by Iron defender Wayne Corden. From the resulting spot-kick Forssell sent keeper Musselwhite the wrong way to give the Blues an unassailable 2-0 lead.
With the second goal killing-off any threats of a cup upset Bruce sent on youngsters Marcos Painter and Peter Till for their first team debuts.