Phil Smith’s work ethic has seen him offered a one-month deal at Pompey.
And caretaker boss Guy Whittingham warned Simon Eastwood that the new recruit is breathing down his neck.
Smith, who was released by Swindon in the summer, has been offered terms with the Blues after arriving on trial this week.
He is expected to be on the bench at Tranmere tomorrow after the Blues’ backroom team decided not to risk going with one keeper in their ranks.
Whittingham explained goalkeeper coach John Keeley felt Smith had shown the right willingness in training this week.
He said: ‘Phil has trained with the rest of the first team and has a good work ethic.
‘John Keeley has worked with him closely in sessions and he has shown a desire to do well.
‘He does his specific stuff with John Keeley and then joins in with the others.
‘He’s an experienced player who knows his body, knows what he is doing and how to train properly.
‘John’s been really pleased with him. It wasn’t worth taking a risk with one keeper, so we decided to go this way.
‘He’s been without a club but I don’t think you lose things mentally. It’s just if the body reacts as quickly when you have time off.
‘But we’ve seen improvements from Monday to now.’
Smith will sign until the new year, with the 32-year-old cover for Eastwood – but that situation could change.
Whittingham said: ‘He’s not here as number one. He’s here as cover. But, as part of that he’s there as a bit of competition for Eastie as well because you can’t have a keeper thinking he’s just going to play the next game.
‘Eastie still has to play well, first and foremost.
‘Phil may not be an immediate replacement but, come the busy Christmas period, who knows what could happen if Eastie isn’t playing well?’
Eastwood will start in goal tomorrow and has been challenged to take his chance.
Whittingham said: ‘The gauntlet has been thrown down to Eastie – and it’s up to him.
‘He’s a down-to-earth lad and has trained hard.
‘He did well when Mikkel (Andersen) wasn’t playing. I have no worries about him.’