Adam Forshaw, fresh from signing a new three-year deal with Brentford, nodded home the opener despite Darnell Fisher's best efforts.
Celtic's strike duo combined 13 minutes later to level things up, though, Amido Balde bundling the ball home after a pass from Watt and 12 minutes after the break McGregor curled home a lovely drive into the bottom corner.
Rangers continued their unbeaten start to their pre-season campaign with a 1-0 win over Dutch second division side FC Emmen.
Irish striker Jon Daly was on target for the Third Division champions as they completed the fifth win of their summer schedule so far.
Former Celtic and Scotland defender Stephen McManus made his first appearance in Motherwell colours as they overcame Morecambe at the Globe Arena.
The central defender missed out on the Fir Park side's friendly defeat to Newcastle on Tuesday night despite penning a one-year deal the day before, but was in the side that won 2-1 over Jim Bentley's Shrimps.
Trialists Henri Anier and Jack Smith struck for the Steelmen - who will face Russian side Kuban Krasnodar in the Europa League third qualifying round next month - either side of Padraig Arnold's first-half equaliser for the League Two side.
At Palmerston, administration-hit Hearts continued to put their financial worries to one side as they ground out a goalless draw with Queen of the South.
Jambos boss Gary Locke, who is barred from signing players as a result of the club's transfer embargo, was again forced to name a youthful side but there was a boost for the Tynecastle manager when injured skipper Danny Wilson signalled he may be nearing full fitness when he joined his team-mates for the pre-match warm-up.
St Mirren, meanwhile, suffered their first defeat to local rivals Morton in eight years as they surrendered the Renfrewshire Cup.
The Buddies struck first through Paul McGowan but were forced to level with Gary Teale's strike after David O'Brien and Kabba-Modou Cham had the Greenock side ahead.
However, the visitors to St Mirren Park finally claimed their first trophy success since 2005 when Reece Hands and Cham netted after the break to confirm a 4-2 scoreline.
Ross County earned a 2-2 draw with a Manchester United XI featuring first-team squad members Federico Macheda and Bebe in Dingwall.
Davide Petrucci wiped out Kevin Luckassen's opener for the Staggies with a free-kick before Alex Cooper put the home side ahead for a second time. However, victory for Derek Adams' side was denied with Ryan Tunnicliffe's leveller 25 minutes from time.
County's highland rivals Inverness were also held to a 2-2 stalemate as they journeyed to Kirkcaldy to take on Raith Rovers.
Stuart Anderson and Calum Elliot had the Fifers two up but Paul Watson's own goal halved the deficit at the break and 18-year-old Liam Polworth restored parity with seven minutes remaining.
Niall McGinn was the goal-scoring saviour for Aberdeen once more as his double against Shelbourne ensured the Reds wrapped up their three-game tour of Ireland with a 100 per cent winning record.
Jake Donnelly had put the hosts ahead midway through the first 45 but two goals inside five minutes from Northern Ireland striker McGinn sealed a 2-1 success.
While the Dons were winning in Dublin, Kilmarnock were victorious in Belfast.
The Rugby Park side were in the Northern Irish capital to face Glentoran, triumphing through Dublin-born frontman Paul Heffernan's double.
Steven Lawless was forced to miss Partick Thistle's trip to Arbroath as his partner was giving birth.
However, on-loan Liverpool striker Henoc Mukendi made his first appearance for the Jags in a 1-0 defeat which saw the SPL new boys go down to Graham Bayne's winner at Gayfield.