Academy product Wagstaff, 22, is the club's current longest-serving player and scored five goals in 34 league appearances last term as the Addicks won the League One title.
Charlton boss Chris Powell told the club's official website: "Scott has been on the fringes so far this season, so instead of him just being in the squad and on the bench and not getting much game time, this is an opportunity for him to get some minutes under his belt.
"He loves playing and deserves to play football every weekend. We will benefit from him playing regularly and getting some much-needed football, and then, at the end of the 28 days, we can assess things further."
