West Brom midfielder Allan has made six appearances since returning for a second spell with Pompey in October, scoring in a home defeat to Leyton Orient.
Striker Jervis has played three times since arriving from Birmingham in November, netting an equaliser in a 2-2 draw against league leaders Tranmere earlier this month.
Both deals run up to and include the League One fixture at Walsall on Friday January 4.






