He should be fine to ride the indoor trainer immediately. He'd just be training at the Tour anyway, so probably wouldn't mind missing it too much. Will be a blow to Philipsen sprinting for the green jersey though.
If I were to give MvdP advice, I'd tell him to bin the entire rest of the road season, only do MTB, (ideally) win the MTB XCO WC and having finally scratched that itch, focus entirely on the classics from 2026 and ride CX, gravel and MTB only for fun and mostly outside of competition or in the second half of the year after the spring classics and the Tour.
He does tend to crash a bunch in MTB and just dedicating himself to it, will definitely help him avoid that and actually give him a proper shot at winning the XCO WC. While it seems everyone at and around Alpecin seem to think he can simply dip in and out and still win, it doesn't actually seem to be happening. I love to see him try, because there is value in leaving your comfort zone and trying hard things, but I'd also love to see him win MSR, RvV & PR another 3-4 times each, so not being injured from MTB would be rather helpful there. 😉
Do only MTB for 2025, win the XCO WC and then road road road road road! 😛