Flintoff's rapid 62 not out, his highest score since he made 89 in his last Test appearance against Australia in Sydney in January 2007, saw Lancashire go second in the table with an eight-wicket win.
It was all the more heartening given his three previous first-class innings were all noughts.
But the 30-year-old, who spent six weeks out with a side injury, is expected to be left out when national selector Geoff Miller announces Thursday the squad for next week's first Test against South Africa at Lord's.
However, that squad will be for that match only, and Flintoff is in line to be considered for the second Test at Headingley which starts on July 18.
Law, who also saw fast bowler Flintoff bowl 34 aggressive yet wicketless overs against Sussex on a docile pitch, said: "He bowled superbly well against Sussex even though he didn't take a wicket on a track that never helped the bowlers.
"And he's a great bloke to have in your dressing room," added Law, a talented batsman restricted to just one Test cap for his native Australia.
"I gave him the opportunity to bat number four in the second innings and he said, 'Give it to me, I want it'," revealed Law, whose first innings 158 put the visitors on course for victory.
"Selfishly, I wouldn't mind if they (England) left him out for the whole series. That would mean him playing more matches for Lancashire.
"But he's a world-class performer. He's a guy for whom you turn up and watch cricket. You do anything to see him bat and bowl."
"I think England have missed him greatly. They have been trying to say they haven't but deep down they know they have."
"From their point of view, I think as soon as he's ready he should play."
However, while England take on Graeme Smith's men at Lord's, Flintoff can expect to be playing for Lancashire against Hampshire at the Rose Bowl.
"It's plain for everybody to see where he's at: the work he has put in has been absolutely fantastic but there is bat form to consider, he will want some runs, and a few more miles in his legs as well," said Miller.
"We have just picked for this first Test match because obviously Fred is in a situation and there is a chance to have a look at that," former England off-spinner Miller, speaking before Flintoff's fifty against Sussex, added.
"We are not sure where we are beyond that first Test match."
When Flintoff last played Test cricket, he was captain of the team beaten in Sydney to condemn England to a 5-0 Ashes whitewash.
For much of the intervening time he has been sidelined with a left ankle injury. The suspect joint, placed under strain by Flintoff's bowling action, has now been operated on four times.
Meanwhile doubts emerged over England captain Michael Vaughan's fitness this week after the Yorkshire batsman suffered a recurrence of the right knee trouble which forced him out of the 2006/07 Ashes.
However, Yorkshire coach Martyn Moxon said there was "no danger" of Vaughan missing the Test match.
England have played the same XI in their last five Tests, winning four of them, and Miller said: "Consistency, that's the watchword we have preached all the way along, and we have just won two Test series against New Zealand, one away and one at home."
Sussex 253&245 lost to Lancashire 392&109/2 (PJ Horton 38 n o, Flintoff 62 n o; JD Lewry 2/34).