Leeds, without 12 members of their first team squad due to injury, were overpowered 23-6, after conceding 11 points in the final nine minutes.
Rhinos have not won at Widnes since 2014 and their four successive defeats there have followed a similar pattern.
“We might as well go back to what I said last year and the year before and the year before that,” McDermott conceded in his post-match press conference.
“There’s always been a bit of a scenario when we’ve lost here over the last few seasons that we’ve got a bit of an injury list and Widnes are coming off the back of some disappointing losses so they’ve got a bit of momentum.
“It went to script again.”
Seven of Leeds’s side were making their first appearances of the season and though the champions’ makeshift line-up gave a good account of themselves in defence, they rarely threatened to break Widnes’s line.
Rhinos led 6-4 at half-time after a hugely spirited defensive effort in the opening 40 minutes.
Widnes were on the front foot for most of the opening period and had enough possession near Leeds’s line to post more points, but the visitors cracked only once.
Rhinos’ determination in defence was highlighted when Rhys Hanbury, a constant attacking threat from full-back, made a clean break with Joe Mellor in support. The stand-off took the pass and had a clear run to the line, but Ashton Golding chased back to make a try-saving tackle.
Gilmore had opened the scoring five minutes earlier, at the end of the first quarter, when he cut through following an offload by Widnes’s Papua New Guinean forward Wellington Albert.
Leeds looked out on their feet for much of the opening period, but rallied to snatch the lead eight minutes before the break when Brad Dwyer forced his way over from acting-half and Kallum Watkins added the extras.
Despite Widnes being reduced to 12 men early in the second half, when Hep Cahill was sin-binned for a foul on Richie Myler, Leeds were second-best after the interval.
Widnes hit the front for the second time on 55 minutes when Krisnan Inu collected Tom Gilmore’s cross-field kick.
He also converted from the touchline and added a penalty goal, but Leeds were just one score adrift until the final 10 minutes.
Danny Craven’s drop goal left Leeds with too much to do and they ran out of steam completely in the final stages, conceding tries to Danny Walker and Inu, whose goal completed the scoring.
McDermott refused to use injuries – or the fact Leeds had returned from their World Club Challenge trip to Australia just a week earlier – as an excuse.
“We haven’t been down a mine digging coal,” he said.
“Everyone talks about the travel but it’s not the worst thing in the world.”
Describing it as a “disappointing loss”, he insisted: “I thought we showed loads of endeavour with the ball, but we didn’t do enough with it.
“We didn’t stretch or challenge Widnes enough, there were way too many one-off carries and too much grunt.
“Widnes don’t spend that much on their squad and they are used to being in battles and want to be in battles. We ended up playing a version of the game that suited them.
“We had enough field position and enough momentum to be able to beat Widnes and we should have done, but they are a tough team who won’t give in.”
Widnes Vikings: Hanbury, Chamberlain, Craven, Inu, Marsh, Mellor, Gilmore, J Chapelhow, Heremaia, Cahill, Dean, Whitley, Olbison. Substitutes: Walker, W Albert, T Chapelhow, Wilde.
Leeds Rhinos: Golding, Newman, Watkins, Keinhorst, Handley, Sutcliffe, Myler, Jones-Buchanan, Parcell, Singleton, Walters, Delaney, Ormondroyd. Substitutes: Dwyer, Garbutt, Oledzki. Unused Smith.
Referee: R Hicks (Oldham).