Learn Russian: Менеджер стабилизировал ситуацию вчера. - The manager stabilized the situation yesterday.

Стабилизировать is a biaspectual verb, meaning it uses the exact same form for both aspects. In this sentence, it functions as a perfective verb because it states a single, completed event with a clear result: the situation was successfully fixed. If the sentence included a duration marker like весь день (all day), the exact same word would act as an imperfective verb describing an ongoing process.

Why doesn't this verb have a prefix like с- or по- to clearly mark it as perfective?

Many foreign loanwords in Russian end in -овать or -ировать and enter the language as biaspectual verbs. While some older borrowings eventually develop perfective pairs with prefixes (like скопировать for копировать), стабилизировать has not. It relies entirely on context, such as stating a factual past result, to convey the perfective aspect.

If the aspect is purely contextual, how do we distinguish between perfective and imperfective in the future tense?

You distinguish them through how you build the future tense grammatically. For the imperfective future (will be stabilizing), you use the compound form with the auxiliary verb: будет стабилизировать. For the perfective future (will stabilize), you conjugate the biaspectual verb directly: стабилизирует.

Does менеджер mean exactly the same thing as manager in English?