Rarely has there been a bigger strain on Avery Bradley’s responsibilities. As the Celtics guard has chased Raymond Felton and J.R. Smith across the floor — the former largely in vain — his offensive numbers have continued to suffer. Bradley has averaged 6.5 points on 11-for-30 shooting (37 percent) over the first four games of the Eastern Conference quarterfinals against the New York Knicks, with 3.7 points on 4-for-16 shooting (25 percent) in the last three. The combination of handling the ball and Bradley’s usual defensive load has clearly had an impact on his offense. But the Celtics don’t have any choice without a true point guard. The C’s also don’t have another option on defense for the rampaging Felton. “Felton was pretty good,” coach Doc Rivers said of the Knicks guard’s 27-point Game 4 performance. “He took the role of J.R. Smith and did a pretty good job of it.” Rest for the weary The extra day between games will help the Celtics, somewhat. Paul Pierce, who played 50 minutes Sunday in the Game 4 overtime victory, Jason Terry, who logged 41 minutes, and Jeff Green, who had a 49-minute run, clearly needed a break before the C’s attempt to cut further into the Knicks’ 3-1 series lead tomorrow night at Madison Square Garden.