The 34-year-old has been without a club for a year after leaving Bolton. However, Alnwick was reportedly wanted by Sunderland during the summer and has more than 250 professional games under his belt.
Alex Bass is currently sidelined with a calf injury and there is concerns that the Blues lack experience behind No1 stopper Craig MacGillivray.
Duncan Turnbull has served on the bench for the past five games but is a rookie, having made just one senior appearance.
Meanwhile, 19-year-old Taylor Seymour falls into the same category despite being well rated.
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Kenny Jackett will be very aware of the Chichester-born keeper as he trialled with Pompey for a few days during the boss' first pre-season. Isted made 11 appearances on loan at National League Wealdstone earlier this season and could prove decent back-up if needed.
The England youth international impressed on loan at AFC Wimbledon last season, making 26 appearances. Trott recently made his Hammers debut in the FA Cup but is fourth-choice in the pecking order. Kenny Jackett's friend Stuart Pearce could help do Pompey a favour, although West Ham would likely want Trott playing if he was to depart.
The veteran 36-year-old signed a two-month deal at the Sky Blues in November after Marko Marosi suffered a fractured cheekbone. However, Marosi's close to returning to full fitness, featuring on the bench in their win over Sheffield Wednesday on Wednesday night, and Camps' departure has now been confirmed. The ex-Norwich man has a wealth of Championship experience.
The Dubliner has never played for the Robins since arriving in the summer of 2019.Still. he has featured in the Championship, League One and League Two during his career and could do a job while Alex Bass is sidelined if required.
A third spell at Pompey for the Irishman, perhaps? Henderson was highly popular first time round before he suffered a luckless season-ending thigh injury on his second debut when arriving on deadline day in January 2018. He won a new one-year deal at Palace in the summer and is very much back-up.
Yes, we know that's a picture of Kenny Jackett - but there are a lack of Chapman pictures about! Nonetheless, the Aussie is someone who Jackett is keeping tabs on. He signed a new deal until 2022 at Huddersfield and has spent time on loan at non-league Ossett United.
The Czech stopper is very much down the pecking order at the Scottish Championship leaders. He played three times for St Mirren on emergency loan earlier this season and Hearts may be open to Zlamal departing again.
Bishop made 39 appearances for Southend as a teenager before earning a move to Old Trafford last January. Ole Gunnar Solskjaer has admitted the 21-year-old could head out on loan, although United would likely want Bishop to get guaranteed minutes.
The 25-year-old came through the ranks at Middlesbrough and had loan spells with the likes of Hartlepool and Stevenage in League Two before penning a short-term deal with Swindon last summer. He played four times for the Robins before leaving earlier this month. Could do a job.
The Swiss stopper signed a short-term deal at Fleetwood in September after his release from Blackburn. Leutwiler featured 18 times for the Cod Army before departing earlier this month. He's been promoted twice in his career.
The well-travelled 34-year-old dropped out of the Football League in the summer when released by Mansfield to join ninth-tier Anstey Nomads. He recently moved to Stamford ahead of their FA Trophy tie with Hereford, although surely he would jump at a chance to return to the professional game even as a back-up.