Nicholas was impressed as the Pittodrie side won their first eight games of the league season - a new club record - as they moved five points clear at the top of the table.
After a worrying dip in results the Dons are back to their early-season form, taking 13 points from the last available 15 in an unbeaten run that goes back to October, with their last defeat against Celtic in Glasgow.
Aberdeen are currently four points adrift having played a game more so the Parkhead team remain clear favourites to come out on top.
Despite that, Nicholas feels McInnes' men can push them after getting over their slump.
He told Sky Sports Now: "They had a really bad five weeks where they were five points clear. Now, all of a sudden, they're four behind having played a game more.
"Hearts are okay. They've done very, very well and are well run now but I think Aberdeen are the biggest threat.
"They've regrouped, come back and they're coming back into a bit of form. Celtic will, in my opinion, still win the league, but I think Aberdeen can certainly challenge them.
"We've got an Aberdeen home game against Celtic coming up live on Sky in midweek early in February so I look forward to that one."
Source: Sky Sports