The 28-year-old has been shortlisted alongside Oxford’s James Henry, Sheffield Wednesday’s Bailey Peacock-Farrell and Cole Stockton (Morecambe) for the prestigious accolade.
It comes after the midfielder’s instant impact at Fratton Park following his free-transfer arrival from Luton in the close season.
In the five league games the former Manchester United trainee has featured in, he has scored one goal and assisted in three as the Blues sit fifth in the table.
To date, Pompey have scored four goals in League One, with Tunnicliffe involved in each of them in some capacity.
To win the August accolade, he’ll need to see off some stiff competition, though.
Henry has netted for goals for the U’s so far this season, including his first hat-trick for 11 seasons against Lincoln.
On-loan keeper Peacock-Farrell has conceded just one goal in five games for Wednesday, while he produced a brilliant penalty save in the 2-0 derby win at Rotherham.
Meanwhile, striker Stockton scored four of newly-promoted Morecambe’s six goals last month, including superb individual efforts at Ipswich and Gillingham when he ran around defenders before finishing.
The winner will be announced on Friday.