The central word here is lokanāthaṁ, which a dictionary says means the lord of the world.
If we search for this & similar terms (here, here & here), they appear found in later texts, including Sanskrit & early Mahayana texts.
Notably, the suffix nātha is found in the Dhammapada, where Sujato translates nātho here different to as he translates nātho in Thig 13.3.
Self is indeed the lord of self,
Attā hi attano nātho,
for who else would be one’s lord?
ko hi nātho paro siyā;
When one’s self is well-tamed,
Attanā hi sudantena,
one gains a lord that’s rare indeed.
nāthaṁ labhati dullabhaṁ.
Attavagga, Dhammapada
In the suttas, it seems, the most common word translated as protector is rakkha
Dhamma surely protects one who practices Dhamma;
Dhammo have rakkhati dhammacāriṁ
Thag 4.10
Looking after yourself, you look after others; and looking after others, you look after yourself.
Attānaṁ, bhikkhave, rakkhanto paraṁ rakkhati, paraṁ rakkhanto attānaṁ rakkhati.
SN 47.19
And how does a mendicant protect?
Kathañca, bhikkhave, bhikkhu rakkhitā hoti?
When a mendicant sees a sight with the eyes, they don’t get caught up
in the features and details.
Idha, bhikkhave, bhikkhu cakkhunā rūpaṁ disvā na nimittaggāhī hoti
nānubyañjanaggāhī.
AN 5.140
All of the above said, there are two suttas AN 10.17 & AN 10.18 using the term 'nātha' found in Thig 13.3.
Mendicants, you should live with a protector, not without one.
Sanāthā, bhikkhave, viharatha, mā anāthā.
Living without a protector is suffering.
Dukkhaṁ, bhikkhave, anātho viharati.
There are ten qualities that serve as protector.
Dasayime, bhikkhave, nāthakaraṇā dhammā.
What ten?
Katame dasa?
Firstly, a mendicant is ethical, restrained in the monastic code,
conducting themselves well and seeking alms in suitable places. Seeing
danger in the slightest fault, they keep the rules they’ve undertaken.
New Concise Pali English Dictionary
nātha
masculine
a protector,patron, lord; a husband; a prop, support; ~ especially the lord, the Buddha
PTS Pali English Dictionary
nātha protector, refuge, help
AN.v.23, AN.v.89; Dhp.160 (attā hi attano n.), Dhp.380; Snp verse 1131
(Nd ii.has nāga) Dhp-a.iv.117; Pv-a.1. lokanātha Saviour of the world
(Ep. of the Buddha) Snp verse 995; Pv-a.42 ■ anātha helpless,
unprotected, poor Ja.i.6 (nāthânāthā rich poor); Pv-a.3 (˚sālā poor
house) Pv-a.65 Cp. nādhati.
Ved. nātha, nāth, to which Goth. nipan (to support), Ohg. gināda
(grace)
Personally, I am inclined towards the translation of 'Lord of the World', which means the Buddha is unaffected by or governs the world around him via governance of mindfulness & wisdom. However, as originally said, Thig 13.3 appears to be a later text using early Mahayana language therefore 'Protector or Saviour of the World' may be more accurate in this context.
Also, the term 'lokanāthaṁ' is found in the later Manorathapūraṇī commentary attributed to Buddhaghosa, here.
duve saccāni akkhāsi sambuddho vadataṃ varo sammutiṃ paramatthañca tatiyaṃ nupalabbhati
The Awakened One, best of speakers, Spoke two kinds of truths: The conventional and the ultimate. A third truth does not obtain.
tattha: saṅketavacanaṃ saccaṃ lokasammutikāraṇaṃ paramatthavacanaṃ saccaṃ dhammānaṃ tathalakkhaṇan ti
Therein: The speech wherewith the world converses is true On account of its being agreed upon by the world. The speech which describes what
is ultimate is also true, Through characterizing dhammas as they
really are.
tasmā vohārakusalassa lokanāthassa satthuno sammutiṃ voharantassa musāvādo na jāyatī ti
Therefore, being skilled in common usage, False speech does not arise in the Teacher, Who is Lord of the World, When he speaks according to
conventions.
(Mn. i. 95)