Liam Feeney's first-half strike booked Millwall's place in the FA Cup Fourth Round draw after edging past League One outfit Preston North End.
However, the tie against League One Preston was most definitely one of those games where The Lions negotiated a potential banana skin and ground out victory, but perhaps not with their most convincing performance of late.
Feeney's 31st-minute deflected effort settled a keenly fought encounter in which Millwall dominated the opening half but then found themselves defending in numbers after the break as Preston pushed in search of an elusive equaliser.
"In the first half we played very well," said Jackett, "We had chances and at the start of the second two very good chances, and one break with a three v two. We perhaps needed to take one of those opportunities, but came out of that period just 1-0 up.
We then had to defend for the last 30 minutes and to some degree that's frustrating. The quality of our break, the first pass out of defence was not necessarily great, and we had to put in a real defensive shift for the last half an hour, which we came through with a good clean sheet."
Millwall showed two changes from the side that kicked off the New Year with a win against Bristol City, with skipper Danny Shittu (knee) and midfielder Liam Trotter (back) both ruled out.
In came Karleigh Osborne to partner Mark Beevers at the heart of the back four, with Josh Wright stepping into the centre of the park alongside captain for the day Jimmy Abdou.
The Lions made a promising start and went close to breaking the deadlock in the opening minutes when, after James Henry's free-kick had been cleared by the Preston defence, the ball fell invitingly for Shane Lowry and his rasping drive was turned round the post by Lilywhites keeper Steve Simonsen.
Dany N'Guessan was unable to connect cleanly after Feeney had threaded a fine ball through to Darius Henderson, who in turn laid it off to his strike partner.
And Feeney was again involved on nine minutes, picking out Henderson with a perfectly weighted pass, but his effort was blocked for a corner.
Henderson went even closer midway through the opening half with a thumping angled volley from Henry's corner that smashed against the underside of the bar and away to safety.
Preston created their first noteworthy effort moments later, but Paul Huntington hooked an effort over the bar from 10 yards when he perhaps could have done better.
However it was Millwall who made the all-important breakthrough 14 minutes from half-time. Henry and Adam Smith combined on the flank, and the latter produced a customary excellent run down the right before picking out Feeney, whose shot took a deflection beyond the reach of keeper Simonsen.
Jackett's men looked to have doubled their tally 10 minutes later when Henry's ball across the face of goal was met by N'Guessan with an audacious flicked effort that was somehow scambled off the line by Huntington.
The visitors broke down the other end and Nicky Wroe fired over, but Millwall saw out the opening half looking comfortably in control.
Unfortunately match-winner Feeney was forced off with a hamstring injury so Shaun Batt was introduced to the fray, and Millwall had a couple of early opportunities in the second-half to put the tie beyond Preston's reach.
Henderson wa sput clean through, only to be brilliantly tackled by Huntington and then saw his goal-bound effort tipped round the post by the agile Simonsen.
Keeper Simonsen was called into action again, palming N'Guessan's shot round the post at full stretch, but then Preston began to get right back into the game and ask questions of the home defence, trying to utilise the pace of former Lion David Amoo at every opportunity.
Preston pushed forward in a desperate attempt to force an equaliser and the visitors should have levelled six minutes from time when Stuart Beavon fired wide from 12 yards.
There was a late scare for The Lions when Graham Cummins had time to chest the ball down and hit a sweet shot that looked destined for the back of the net, but Lions stopper David Forde had other ideas and produced a fantastic block to finally kill Preston's Cup dream.
Millwall: Forde, A.Smith, Lowry, Abdou, Osborne, Beevers, Feeney (Batt, 46mins), Wright, Henderson (J.Smith, 86mins), N'Guessan, Henry. Subs not used: M.Taylor, Dunne, Marquis, Racon, Wilkinson.
Booking: A.Smith (67mins)
Scorer: Feeney (31mins)
Preston NE: Simonsen, Huntington, Robertson, Cansdell-Sherriff (Byrom, 46mins), Wroe, Kane (Amoo, 46mins), Buchanan, Welsh, Mousinho (Cummins, 81mins), King, Beavon. Subs not used: Stuckmann, Proctor, Beardsley, Hayhurst.
Att: 5,364 (away 468)