Skipper Paul Butler and defender Matthew Kilgallon are suspended for the trip to Fratton Park, while Butler also misses the league game against Wigan on Sunday.
Therefore, Johnson and Keogh have been recalled from their loan spells at Sunderland and Scunthorpe respectively and they are due to be named among the substitutes tomorrow night.
Jamie Winter has also travelled down to the club's training camp on the south coast but Jermaine Wright (Achilles) and Stephen Crainey (groin) are still missing.
Therefore, Johnson and Keogh have been recalled from their loan spells at Sunderland and Scunthorpe respectively and they are due to be named among the substitutes tomorrow night.
Jamie Winter has also travelled down to the club's training camp on the south coast but Jermaine Wright (Achilles) and Stephen Crainey (groin) are still missing.