Just when Mandaric thought things couldn't get any worse, he watched a hapless first-half display from his team which looked set to put the Baggies on their way to their first away win of the season.
But LuaLua and fellow substitute Eyal Berkovic's introduction in the second half prompted a stunning late turnaround to leave Albion manager Bryan Robson seething at the final whistle.
Berkovic, on as an 80th minute replacement for Nigel Quashie, delivered an inviting free-kick for captain Arjan De Zeeuw to head Pompey level before LuaLua sparked scenes of mass celebration with his first goal of the campaign a minute from time.
Berkovic was once again the provider as he beat West Brom's offside trap with a delightful throughball for the Congo striker to control and steady himself before rifling a shot into the roof of the net.
Portsmouth had made the worst possible start when Dejan Stefanovic put through his own goal after only 14 minutes.
Baggies full-back Neil Clement tormented Andy Griffin on the left-hand side before delivering a low cross which the hapless Stefanovic could only side-foot past Jamie Ashdown.
David Unsworth was unlucky not to equalise four minutes later with a superb piece of individual skill after being picked out on the edge of the area by lone striker Ricardo Fuller's deep cross from the right.
The defender controlled his centre before hitting an instinctive left-foot lob which had goalkeeper Russell Hoult beaten only to bounce off the top of the bar.
But Portsmouth didn't have to wait long for their equaliser as Stefanovic redeemed himself after his earlier error.
The Serbian, having been switched to the left after Unsworth limped off with an injury, produced a dangerous cross in the 34th minute which West Brom centre-back Darren Purse could only slice into the back of his own net.
Both sides would have been hoping that was an end to the defensive mistakes but Pompey then contrived to hand their opponents the lead again on the stroke of half-time.
In his haste to prevent the ball going out for a corner, Matthew Taylor succeeded in giving away possession to Riccardo Scimeca to cross for Robert Earnshaw to be left with the simple task of side-footing home his fifth goal in five games.
Patrik Berger wasted two good shooting opportunities to level the scores for Portsmouth after the interval and it seemed they were heading for defeat until the intervention of LuaLua and Berkovic.
MAN OF THE MATCH: Ricardo Fuller (Portsmouth) - Had a thankless task on his own up front but led the line superbly.