Last night’s impressive 3-1 win, courtesy of a John Marquis brace and excellent Tommy Rowe goal, moved Rovers ten points clear of fourth-placed Pompey.
The victory also stretched Doncaster’s unbeaten league run at the Keepmoat Stadium to 16 games.
“Time will tell how significant that result was,” said Ferguson afterwards.
“For me, with 21 games to go ten points is catchable.
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“What we’ve done is maintain the home form and the home record which is proving to be really crucial.
“And in the second half, in particular, we showed we’re a good team.”
Marquis headed Rovers ahead after just five minutes but Pompey grew into the game and levelled before the break through Kal Naismaith.
Rowe put the hosts back ahead when he finished off a slick move before Marquis sealed victory with a volley.
“Half time came at the right time for us,” said Ferguson.
“The problem was that we didn’t keep the ball well enough.
“If you keep giving the ball back to Portsmouth they’ll cause you problems because they’re a good technical team.
“In the second half we kept the ball much, much better, which allowed our centre halves to get up the pitch and we could go and press them.
“That was the big difference and the second goal was a really good example of that.
“The second half was much better.
“We were much more like ourselves after half time and we were much more composed on the ball, and that won us the game.”
Goalkeeper Marko Marosi, stretchered off in the closing stages, will undergo a scan on his ankle after landing awkwardly.