The 19-year-old Slovakian Under-21 international joins the Blues on a youth deal up to and including Saturday, February 22.
Rusnak had a brief spell with League One side Oldham Athletic earlier this season, making three appearances for the Latics.
He is yet to make an appearance for the Man City first team and will wear the number 35 for Birmingham.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Liverpool | 7 | 6 | 0 | 1 | 13 | 2 | 11 | 18 |
| 2 | Manchester CityMan City | 7 | 5 | 2 | 0 | 17 | 8 | 9 | 17 |
| 3 | Arsenal | 7 | 5 | 2 | 0 | 15 | 6 | 9 | 17 |
| 4 | Chelsea | 6 | 4 | 1 | 1 | 15 | 7 | 8 | 13 |
| 5 | Aston Villa | 6 | 4 | 1 | 1 | 12 | 9 | 3 | 13 |
| 6 | Newcastle UnitedNewcastle | 7 | 3 | 3 | 1 | 8 | 7 | 1 | 12 |
| 7 | Fulham | 7 | 3 | 2 | 2 | 10 | 8 | 2 | 11 |
| 8 | Tottenham HotspurSpurs | 6 | 3 | 1 | 2 | 12 | 5 | 7 | 10 |
| 9 | Brentford | 7 | 3 | 1 | 3 | 13 | 13 | 0 | 10 |
| 10 | Brighton & Hove AlbionBrighton | 6 | 2 | 3 | 1 | 10 | 8 | 2 | 9 |
| 11 | Nottingham ForestNott'm Forest | 6 | 2 | 3 | 1 | 6 | 5 | 1 | 9 |
| 12 | West Ham UnitedWest Ham | 7 | 2 | 2 | 3 | 10 | 11 | -1 | 8 |
| 13 | Bournemouth | 7 | 2 | 2 | 3 | 8 | 10 | -2 | 8 |
| 14 | Manchester UnitedMan Utd | 6 | 2 | 1 | 3 | 5 | 8 | -3 | 7 |
| 15 | Leicester CityLeicester | 7 | 1 | 3 | 3 | 9 | 12 | -3 | 6 |
| 16 | Everton | 7 | 1 | 2 | 4 | 7 | 15 | -8 | 5 |
| 17 | Ipswich TownIpswich | 7 | 0 | 4 | 3 | 6 | 14 | -8 | 4 |
| 18 | Crystal Palace | 7 | 0 | 3 | 4 | 5 | 10 | -5 | 3 |
| 19 | Southampton | 7 | 0 | 1 | 6 | 4 | 15 | -11 | 1 |
| 20 | Wolverhampton WanderersWolves | 7 | 0 | 1 | 6 | 9 | 21 | -12 | 1 |
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