Plus Japan being a growing industrial power could still partly play the role it had OTL during ww1 as a neutral or distant ally: giving money and weapons plus clothes and others to the Allies. This also completely secure New Zealand and Australia plus the British troops fighting in Burma OTL: we're talking of the equivalent of two field armies here, plus the Dutch who have more ships and secure their oil to the Allied war effort (and can use the KNIL to create an Army Corps) while Indochina will probably become Free French like the other colonies in 1943 (meaning the equivalent of an Infantry Division plus tons of rice and rubber).
Plus of course the entire US Navy, Air Force, Marines and US Divisions now being sent in Europe like said in previous post.
If a distant ally, Japan will probably act like during WW1: protecting convoys? sending destroyers and light cruisers, submarines and air squadrons? No problem but I don't think Japan will be willing to send its big units or Army Divisions (or perhaps its older battleships and Battlecruisers and old carriers, in which case the Royal Navy power just doubles in size, and for Army units, perhaps to secure the Middle East, serve as occupation troops, freeing european manpower for the frontline).
In other words, US declare war in early 1942 against Germany and Italy and Japan being neutral or vaguely ally has huge indirect consequences. No problem for landing operations and the lack of landing ships for one.