The England international opened the scoring at Old Trafford as he continues his most successful goal-scoring season of his career.
The 21-year-old now has 13 goals in all competitions after striking three times in his last four outings for United.
England and Newcastle legend Shearer believes that the forward is now making the difference for his side, and he has explained how.
‘It was a beautiful finish from Rashford for the goal,’ Shearer said on Match of the Day. ‘He is in charge of the situation, it was a good first touch, he doesn’t panic and he knows what he wants to do. It was so calm.
‘What I like about Rashford is he points to where he wants the ball playing into his path. He made the run in behind several times and that was the difference in the first half.
‘Rashford was the threat and he was the difference.’
United moved back into the top four in the Premier League with their victory over the Hornets.
Anthony Martial scored their second goal, before Abdoulaye Doucoure pinched a 90th minute consolation for the visitors.
Arsenal have the opportunity to overtake the Red Devils on Monday night when they host Newcastle, as the battle for the Champions League places looks like it will go right down to the wire.
United are back in action away at Wolves on Tuesday night.