But Kristoffer Ajer netted two minutes later and Celtic took control in the second half after a Leigh Griffiths free-kick edged them in front four minutes before the interval.

James Forrest scored midway through the half and Odsonne Edouard and Ryan Christie came off the bench to make it 12 goals in two Ladbrokes Premiership games, the latter from the spot before Donnelly grabbed his second in stoppage-time.

Lennon said: "The only source of frustration was the way we started the game, we invited pressure we didn't need to invite and conceded a poor goal.

"Once we eradicated that and started playing the ball forward quickly we were absolutely outstanding.

"I know from my own experience how difficult it can be coming straight off the back of a Champions League trip but we looked really powerful.

"There's a ruthlessness about us at the moment, good power in the team, good pace. Five goals away from home at any stage of the season you have got to be pleased, but particularly in the circumstances on the back of a really tough night in Romania."

Lennon made four changes, two enforced in defence through suspension for Jozo Simunovic and a minor muscle problem for Hatem Abd Elhamed.

Nir Bitton and Christopher Jullien slotted into central defence with Ajer impressing at right-back, and both Griffiths and Olivier Ntcham proved more than able deputies for Edouard and Christie.

"I don't have a first-choice 11, I shouldn't have," Lennon said. "These guys are quality players, it's not like they are back-up players. These are first-team players – Griffiths is a Scottish international with a proven track record of goals, and Ntcham is a £5million midfield player who has shown he is a £5million player, probably more. I had no hesitation in playing them.

"We had to change the back four with Hatem and Jozo being out and that might have caused us a few teething problems early on. But as the game grew we looked very assured and strong."

Motherwell manager Stephen Robinson bemoaned the way his side caused their own downfall.

"Four out of the five goals were individual mistakes," Robinson said. "Some of our football was terrific first half, we pressed the life out of them.

"I thought we dominated the game in the first half but if you defend like that against the calibre of team like Celtic you are going to get punished. So it's a very big learning curve.

"The first goal was indecision from Taity (Richard Tait), the second goal we give away a free-kick from a long punt up the pitch. It was a bad decision for the fourth and then we lose the ball before the penalty for the fifth.

"Those things we can take out, that's the positive, but I thought there were a lot of good performances."