Lutut saya perlu direhatkan hari ini kerana saya terjatuh semalam.

In Malay, possession usually works as noun + possessor.

So:

• lutut saya = my knee
• rumah saya = my house
• kereta dia = his/her car

This is completely normal Malay word order. English puts the possessor first, but Malay usually puts it after the noun.

By itself, lutut can refer to a knee or knees, depending on context. Malay nouns usually do not change form for singular vs. plural.

In this sentence, the context makes it sound like one knee.

If you wanted to be clearly plural, you could say:

• kedua-dua lutut saya = both my knees

So lutut saya is natural, and context tells you whether it is singular or plural.

Perlu means need to, needs to, or necessary.

In this sentence, it shows necessity:

• Lutut saya perlu direhatkan = my knee needs to be rested

It is a common and useful word. You can also use it with verbs directly:

• Saya perlu tidur = I need to sleep
• Kita perlu pergi sekarang = we need to go now

So perlu is a very common way to express need or necessity in Malay.

Direhatkan can be broken down like this:

• rehat = rest
• di- = passive marker
• -kan = often makes the verb mean something like cause to or put into that state

So merehatkan means to rest something or to give something rest.
Then direhatkan is the passive form: to be rested or to be given rest.

That is why:

• Lutut saya perlu direhatkan means the knee needs to be rested

A very close active version would be:

• Saya perlu merehatkan lutut saya = I need to rest my knee

Both are possible, but they emphasize different things.

• Lutut saya perlu direhatkan focuses on the knee
• Saya perlu merehatkan lutut saya focuses on me as the person doing the action

Malay uses passive quite naturally, often more naturally than English does. In this sentence, the speaker is highlighting the condition of the knee, so the passive sounds very reasonable.

This is a very important distinction.

• jatuh = fall
• terjatuh = fall accidentally / end up falling

The prefix ter- often shows that something happened unintentionally or unexpectedly.

So:

• Saya jatuh = I fell
• Saya terjatuh = I accidentally fell / I had a fall

In this sentence, terjatuh is a natural choice because falling is normally not intentional.

Yes, you often can.

• kerana = because
• sebab = because / reason

Kerana tends to sound a bit more standard or formal.
Sebab is very common in everyday spoken Malay.

So these are both understandable:

• ... kerana saya terjatuh semalam
• ... sebab saya terjatuh semalam

The version with kerana is slightly more neutral or written-style.

This word can confuse English speakers.

Very often, semalam means yesterday. In some contexts, it can also be understood as last night, but if you want to be clearly specific about last night, Malay often uses:

• malam tadi = last night

In this sentence, semalam is naturally understood as yesterday.

Yes, Malay time expressions are fairly flexible.

Here:

• Lutut saya perlu direhatkan hari ini ...

means the time phrase hari ini modifies the main statement.

You could also say:

• Hari ini, lutut saya perlu direhatkan ...
• Saya terjatuh semalam, jadi lutut saya perlu direhatkan hari ini

Likewise for semalam, you can say:

• Saya terjatuh semalam
• Semalam saya terjatuh

Both are natural. Position often depends on emphasis and style.

Yes, but it is slightly different in feel.

• Lutut saya perlu berehat = my knee needs to rest
• Lutut saya perlu direhatkan = my knee needs to be rested

The second version suggests that the knee should be given rest by the person managing it. That often sounds more natural when talking about an injured body part.

Another very natural alternative is:

• Saya perlu merehatkan lutut saya

So all three are possible, but direhatkan gives a nice sense of the knee needs rest.

In everyday Malay, rehat is very flexible. It is often treated like rest / take a break, and with affixes it clearly becomes verbal.

For example:

• rehat = rest / break
• berehat = to rest
• merehatkan = to rest something
• direhatkan = to be rested

So in this sentence, you are seeing the affixed verbal form direhatkan.

It is natural and understandable. It sounds fairly standard and a little careful in style.

In more casual speech, someone might also say:

• Saya kena rehatkan lutut saya hari ini sebab saya terjatuh semalam
• Lutut saya kena direhatkan hari ini sebab saya jatuh semalam

A few differences there:

• kena is very common in speech for have to / need to
• sebab is more conversational than kerana
• jatuh may replace terjatuh in casual speech

So your original sentence is good, especially for clear, standard Malay.