No, the quarterback or offensive captain has to notify the referee in order for an offensive lineman to be an eligible receiver. The exception is if the offensive lineman to be eligible is also an offensive captain.
This is found in the NFL rulebook under Rule 8, Section 1, Article 4 (in part, emphasis mine).
The quarterback or offensive captain informs the referee that a
usually ineligible receiver will be eligible for that play.
Why does a player who is lined up in a legal position to receiver need to give an additional signal the opposition that they are eligible?
The player in question is an offensive lineman (center, guard, or tackle).
Rule 5, Section 1, Article 2 (in part, emphasis mine) states:
All players must wear numerals on their jerseys in accordance with
Rule 5, Section 4, Article 3(c). Such numerals must be by playing
position, as follows:
(a) quarterbacks, punters, and placekickers: 1-19
(b) running backs and defensive backs: 20-49
(c) centers: 50-79
(d) offensive guards and tackles: 60-79
(e) wide receivers: 10-19 and 80-89
(f) tight ends: 80-89
(g) defensive linemen: 50-79 and 90-99
(h) linebackers: 50-59 and 90-99
Using this rule with an elaboration with Rule 8, Section 1, Article 4 (in part, emphasis mine), offensive linemen (50-79) do not have the numbers of eligible players (1-49 and 80-89). Therefore, offensive linemen must be legally reported to be eligible receivers and line up at the end of the line or at least one yard behind the line.
Offensive players who are on either end of the line, provided they
either have the numbers of eligible players (1-49 and 80-89) or have
legally reported to play a position on the end of the line. See 5-1-2.
Offensive players who are legally at least one yard behind the line at
the snap, provided they either have the numbers of eligible players
(1-49 and 80-89) or have legally reported to play a position in the