The summer signing from Ipswich delivered a fine Fratton Park debut as Pompey comfortably defeated Birmingham in the Carabao Cup first round last night.
Harrison's double either side of half-time helped inspire Kenny Jackett's men to a 3-0 victory.
After spearheading Pompey’s attack throughout pre-season, the £1m-plus arrival of Marquis last week meant Harrison had to settle for a spot on the bench for the League One curtain-raiser at Shrewsbury on Saturday.
Marquis – who netted 26 times for Doncaster last season – missed the Birmingham tie, though, after his partner went into labour.
It handed Harrison an opportunity and he grasped it.
The affable Welshman is hoping to retain his starting berth when Tranmere travel to the south coast on Saturday.
But Harrison won’t be sulking if he again finds himself among the substitutes.
He said: ‘It’s a team game. I can’t stress enough how important the whole squad is.
‘It doesn't matter if I score or someone else does. If John had scored a hat-trick on Saturday then I'd have been buzzing.
‘There’s no bitterness towards anyone.
‘It was disappointing but it wasn’t really surprising in terms of him coming in.
‘I would have played John in terms of him scoring 26 goals last season – that’s coming from me!
‘We’ve got four great strikers at the club. It’s not just John because we have Hawks (Oli Hawkins) and Pits (Brett Pitman) as well.
‘There's competition and it’s healthy. If you get complacent, you can stop working as hard.
‘All four of us are trying to play for that one shirt. We’re always on it in training and it’s good to have that competition.
‘It didn't surprise me when we were linked with John because those are the percentages we need to get promoted into the Championship – he is a great addition.’
Harrison broke the deadlock against Birmingham on the half-hour – although the ball only just crossed the line.
He then added his second in the 54th minute with another headed effort.
Ben Close was also on target with a superb volley.
Although Pep Clotet’s complained Harrison’s first goal shouldn’t have stood, he was always confident.
The ex-Bristol Rovers man added: ‘I knew it was over the line – the defender has headed it on to the bar.
‘I’ve had a few of them disallowed in my career but I'm glad the linesman spotted it.
‘My second showed the quality of the wingers and full-backs in this team to be able to pick you out.
‘Ronan put a good ball in a good area at the back post for me to head home.’