He said in an interview for Slovenian media he felt pain in a muscle behind his knee. In an interview after Itzulia. I don't know if any other information beyond that was shared. Now if we take just this information and guess a bit. Muscle injuries in this area can in general take from one to three weeks or from one to three months to heal properly. Depending on how serious the injury was. In any case i feel that it's a good thing to skip this two races in April and hopefully after some time off the bike for the problem to resolve itself. Likely this involves no or only some light training in May.
In L'Equipe, according to their article he seems healed after one week of treatment but Jumbo's staff prefer to let him rest before going to an altitude camp in Sierra Nevada on April 25th: Wout Van Aert disputera Liège-Bastogne-Liège mais pas Primoz Roglic - L'Équipe (lequipe.fr)
Roglic se remet d'une douleur au genou
Son coéquipier Primoz Roglic, qui devait prendre le départ de la Flèche wallonne mercredi, puis de Liège-Bastogne-Liège dimanche, doit en revanche renoncer. Gêné sur la fin du Tour du Pays basque (4-9 avril) par une douleur musculaire derrière un genou, il semble rétabli après une semaine de soins mais le staff de Jumbo-Visma préfère le laisser au repos avant un stage en altitude, en Sierra Nevada, à partir du 25 avril.
Based on previous years, that Sierra Nevada altitude training in May usually precedes going to Tignes (although doing the Dauphiné might shake things up a bit).
So we can hope it's not too bad.