And he has backed the Coventry striker to ‘easily’ net double figures in the 2022-23 campaign, irrespective of his destination.
His January arrival could potentially have led to a permanent arrangement, yet such plans have long been scrapped.
However, O’Brien has seen enough of the 25-year-old to recognise his talents.
And he is adamant his former team-mate can rediscover his scoring touch with a change of scenery.
O’Brien told The News: ‘Strikers are human beings, we all get affected in different ways in different types of areas, off the pitch, on the pitch.
‘Some can take criticism, some can’t, some like criticism, some like to be told they aren’t good enough and thrive off that. Everyone is different in life.
‘In terms of football, as an attacker there’s nothing better than scoring goals because that gives you the confidence going into the next game.
‘If you keep scoring you end up just finding the ball and putting it into the back of the net, whereas if you haven’t scored in a while you end up going in search of it, vacating the positions you could score from.
‘I’ve realised in my career as an attacker that you just need a few goals to settle yourself in – and luckily I got that early at Pompey.
‘Tyler is a terrific player, I have trained with him for four months, his finishing is top class, he trains really well, he’s a very, very good player and a nice guy.
‘No doubt, mark my words, he will be banging in goals again next season because I believe in him – and he's a great player.
‘One season doesn’t define you. You have seen in the past that it takes one season, one season for a breakthrough year, and players are getting their big move to the Premier League.
‘I have no doubt in my mind that Tyler will easily score double figures next season, I have already told him that.’
In contrast to Walker, O’Brien netted five times and swiftly established himself as a Pompey fan favourite after joining in January.
He added: ‘Sometimes it doesn’t work. You can try as much as you want, but sometimes it doesn’t work.
‘It happened to me at Sunderland, I tried all sorts, I tried my best, I trained every day as hard as I could, even when I was scoring goals I was still getting dropped.
‘Some things just don’t work and you've just got to go to a new place, a new environment, and just start again.
‘Tyler will be back firing next season, 100 per cent. Wherever he goes, Coventry or somewhere else, mark my words he will be scoring.’
A message from the Editor, Mark Waldron
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