The fixture on London’s synthetic pitch, at Ealing Trailfinders, was McGuire’s 13th appearance of an injury-hit first year as Leeds’ on-field leader.
He suffered knee damage in the opening game of the campaign, in February, a quad injury two months later and an ankle problem in July. With only four games remaining, there is now a question mark over whether he will play again this season.
The injury was being assessed today, but Leeds coach Brian McDermott said: “I don’t think it’s mega-bad, but I think it’s a similar type of injury he had when he was out with his ankle last time.
“We are looking at some weeks there, I think.”
Joel Moon, normally a centre, started at stand-off against London and teenager Jordan Lilley scored a try from scrum-half after coming off the bench late in the first half to replace McGuire.
So Leeds do have cover, but the captain’s latest setback is a concern with their place in Super League for next year still not guaranteed.
McDermott admitted McGuire will be missed.
“He has played less than half (Leeds’ games) for us this year,” the coach pointed out.
“When he has played he has been great.
“It is great to have him in the team and he has worked really well with Rob (Burrow) this year.
“When he has played we’ve been a great team and when he has not we look a bit clunky and disjointed.
“We certainly looked that when he went off.”
Clunky and disjointed was a fair description of Leeds’ overall performance against the team who finished second in the Championship.
London led twice in the first half and scored three of the game’s final four tries.
Leeds only really asserted their dominance in a 15-minute spell either side of half-time, when they moved up through the gears to run in four tries without reply.
Their star turn was provided by Liam Sutcliffe, who began the season as McGuire’s half-back partner, was also tried in the three-quarters, but has thrived since being moved to full-back.
He linked nicely into the line, coped well under the high ball in difficult, blustery conditions and finished with 14 points from a try and five goals.
Overall though, Leeds could be satisfied with the victory, extending their 100 per cent record in the middle-eights from three games, but not the performance.
London scored five tries and their 28 points was the most against Leeds since the end of May, when they conceded 52 at Warrington Wolves.
London opened the scoring through Rhys Williams after just five minutes and went 12-10 in front when James Cunningham ghosted through from acting-half.
Those tries were separated by the first of winger Tom Briscoe’s hat-trick and Sutcliffe’s touchdown. Briscoe went over again soon after Cunningham’s try, but Leeds’ nerves were jangling until James Segeyaro finished off a fine move two minutes before the break.
Lilley, Mitch Achurch and Watkins – who had earlier dropped the ball over the line – crossed early in the second half and Briscoe completed his hat-trick after Nick Slyney went in for London.
The hosts finished with a flourish as Iliess Macani and Elliot Kear went over in the final moments and Jamie Soward took his tally of conversions to four.
London Broncos: Walker, Williams, Hellewell, Kear, Macani, Soward, Barthau, Slyney, Cunningham, Ioane, Harrison, Garside, Bussey. Substitutes: Ackers, Thackeray, Battye, Offerdahl.
Leeds Rhinos: Sutcliffe, T Briscoe, Watkins, Keinhorst, Hall, Moon, McGuire, Galloway, Segeyaro, Garbutt, Ferres, Ablett, Jones-Buchanan. Substitutes: Lilley, Achurch, Mullally, Cuthbertson.
Referee: C Campbell (Widnes).