Kenny Jackett has lifted the lid on the substitution which earned Pompey an unlikely Doncaster victory.
The Blues boss elected to bring off John Marquis at the interval, no doubt galling for the striker upon his return to the club where he netted 67 times in 153 appearances.
With the match in stoppage-time, Harrison produced a superb looping header from James Bolton’s cross to snatch a last-gasp 2-1 triumph.
A fifth goal of the season for the former Ipswich man, who continues to stake an excellent argument for a starting spot.
Jackett said: ‘I didn’t think in the first half we had enough of a threat down the sides. If teams are going to play open, you need that one right up the middle who can get in with one pass.
‘Maybe it was the right day for Ellis, he responded with a great header and it was a winner as well.
‘Nothing against John, I felt that perhaps Ellis could help us to be more of a threat and more incisive, because we weren’t good enough in the first half and didn’t have enough shots.
‘He’s good in the air, can be a handful, has pace and power, and is slightly different to Brett and John, who are maybe technique based.
‘For Ellis, though, he showed a good complement and I am delighted for him to be able to come on like that and get the winner.
‘In terms of challenging for that number nine position, it’s a big one for him, most of his goals so far have been headers. That was another terrific header on Saturday.
‘If we can get the service into him, he certainly has the pace and power to upset defenders.’
Provider of the decisive goal was full-back Bolton, with a delivery from the right.
The former Shrewsbury defender also created Brett Pitman’s winner against Bolton the previous match.
Jackett added: ‘That’s two assists for James, creating two winners during the last two games, while he also made some fantastic blocks and headers at the far post.
‘That will really help him after his struggles through inquiry.’