LONDON — An unusually busy theater month in London has seen some very unusual recasting: Not often do West End takeovers encompass an Oscar-winning stage neophyte in one instance, an esteemed classical actor turned musical first-timer in the other.

But for different reasons, the appearances of both Matt Damon in “This Is Our Youth,” at the Garrick, and Alex Jennings, the new Henry Higgins in “My Fair Lady” at Drury Lane, merit a return visit to those shows, whatever shortfalls may have occurred around them. (Damon takes at least 12 days off starting May 30 to fulfill publicity duties on “The Bourne Identity,” his next big film.)

The Hollywood actor’s gleaming-toothed presence animates Kenneth Lonergan’s play in a way that wasn’t the case when director Laurence Boswell’s production first opened March 15. London’s original Dennis, Hayden Christensen, was a wiry, edgy, angry young man with a Brando-esque bray whose performance was as brave as it was controversial: Various theatergoers of my acquaintance didn’t like it at all, instead giving themselves to the endearing haplessness of Jake Gyllenhaal’s no less wounded Warren, Dennis’ buddy. This time around, with Damon and Casey Affleck playing the Upper West Side druggies adrift in the Reagan era of two decades ago, one’s sympathies are more equally balanced.

Affleck doesn’t possess anywhere near Gyllenhaal’s force field of charisma, though the character’s “Norse horseman” routine is still priceless. (And Affleck partners well with Summer Phoenix, all pleading, anxious eyes as the reluctant Jessica, Anna Paquin’s previous role.) But because Warren is everyone’s favorite layabout, it makes sense to cast an outright charmer in the less likable part of Dennis. Boasting that nine out of 10 people can’t even hold his gaze, Damon’s Dennis sustains a winning narcissism that Christensen’s Method-style perf didn’t even attempt. Damon, too, gets an inevitable laugh announcing Dennis’ dreamed-of renown as a movie actor-director: “I’d be a genius at it.”

As a result, a play steeped in unhappiness now has a more buoyant feel, with the audience doing its bit by wolf-whistling Damon’s thighs at the start of act two. (By contrast with Christensen’s grunge, Damon looks pretty scrubbed for someone said to have “a cleanliness problem.”) If I preferred the first trio, it’s because Christensen imparted a live-wire element of danger — his second-act breakdown was far more moving — that has since been dulled. On the other hand, it’s about the happiest assignment a theater critic could have to be able to keep comparing casts in so fine a play.

“My Fair Lady,” by contrast, seems less essential on the West End than it did 14 months ago at the National. Perhaps it’s simply that the more didactic elements of Trevor Nunn’s production — during “Show Me,” Eliza grabs a suffragette’s placard stating, “Stop abuse now” — pall with repetition. So does Dennis Waterman’s growly Doolittle, a company holdover who clearly looks bored.

Newcomers Malcolm Sinclair, as a cartoonish Pickering, and Peter Prentice, reprising the traditional take on Freddy as a total twit that Mark Umbers gloriously redefined, don’t really fill the vast Drury Lane stage. Nor does a thin-voiced Joanna Riding, a good actress whose hard-working perf doesn’t quite find Eliza’s bravura gusto.

None of these cavils, however, should keep anyone from checking out Jennings, whose crisply spoken, deeply felt and very well sung) Higgins brings a little-boy-lost quality to the fierce professor. At its best, Nunn’s staging has always addressed two parallel educations — Eliza’s into the King’s English and society, alongside Higgins’ into humanity itself. “I can do without her; I can do without anybody,” Higgins proclaims, Jennings suggesting a Shavian Bobby from “Company” who is, quite simply, scared to death of being alive.

The actor’s own youthfulness — he’s barely a decade older than Riding — allows, too, for a sense of sexuality reined in: This is a Higgins used to lashing out before anything nearly as messy as attraction can be admitted. He’s both the brilliant phonetician and the flawed, incipiently feeling human being, both of which Jennings communicates as naturally as, well, “breathing out and breathing in.”