United have only lost once in 11 games since McNamara left promotion-bound Partick Thistle for Tannadice just before the end of January.
That defeat came at Celtic Park but McNamara and United have a chance to atone when they face Lennon's side at Hampden on Sunday in the William Hill Scottish Cup semi-finals.
United will go into the match on a high after Rory Boulding's injury-time strike earned them victory over Aberdeen and sent them into the top six of the Clydesdale Bank Premier League seconds before the spilt.
"Jackie did very well at Thistle," Lennon said. "He was an intelligent footballer and an intelligent guy and I can see why Dundee United went for him. He's starting to stamp his own personality and ideas on the way Dundee United play now.
"They had a great win last weekend and they'll be buoyant coming into this game off the back of that. They've had a decent run under Jackie and I think they've only lost one game and that was to us."
Celtic have lost their past three matches at Hampden, including last year's semi-final against Hearts, so there is little chance of them taking the game for granted.
And Lennon is determined to lead his team closer to the double after a 3-0 victory over Hibernian last weekend left them one point away from the SPL title.
"Semi-finals are always difficult, no matter who you're playing, as we know from previous experience," the former Celtic captain said. "We've had a good week, we're looking forward to the game and the players are ready to go. The players have worked very hard this season and I'm hoping they get what they deserve.
"We played very well against Hibs last weekend and I would like to replicate that in the next two games and finish off the season deservedly as champions and in a cup final as well."