NBefore i go on to the injuries which befell both women, let me talk a little of the competition for both women at these times. First, Seles was facing Steffi Graf and Navratilova. Serena also had strong competition, perhaps the strongest across the board, but Venus was no Steffi.
Venus of 2002 and 2003 >>> Graf of 1991 and 1992. Venus in 2002 and 2003 reached the finals of 5 of the 7 slam tournaments she played. Graf in 1991 and 1992 reached the finals of only 3 of the 8 slam tournaments she played, 2 of those Wimbledons where all the other players of her era were completely useless, including Monica Seles. Graf in 1991 and 1992 also lost 7 of 8 matches to Gabriela Sabatini, lost a slam semifinal 6-2, 6-0 to Sanchez Vicario, lost 3 times to Jana Novotna, lost 2 of 3 matches to a very old Martina Navratilova who she had not lost to in over 4 years prior to that point. The Venus of 2002-2003 would not have had such results vs these players. Yes peak Graf of 88-89 or 95-96 is probably better than any Venus, and Graf overall had a far greater career than Venus, but that is obviously not the Graf which Seles faced during her dominance, while it was the Venus that Serena faced during hers. Had Serena not existed, 2002 and 2003 would have been the best 2 years of Venus's career. Had Seles not existed, 1991 and 1992 would have been the 9th and 10th best years of Graf's career, basically like they were even with her. Graf only lost 1 match to Seles in 1991-1992, so Seles was quite irrelevant to her extremely mediocre (for her) results at the time. As a rather poor Graf was not even reaching Seles in most of the slams she won during this timeframe, lets instead look at who they actually DID beat in slam finals, plus Serena's one very narrow semifinal defeat which cost her an additional slam to bring both up to 6, in their 2 most dominant years, to get a real idea who faced the tougher competition across the net:
Seles 1991 Australian Open Novotna vs Serena 2002 French Open Venus. Edge Serena. Yes even Venus on clay is much better than Novotna on rebound ace where she had horrendous results the remainder of her career and even skipped from 96-98.
Seles 1991 French Open Sanchez Vicario vs Serena 2002 Wimbledon Venus. Edge Serena. Sanchez was a great clay courter, but obviously not the level of Venus as a grass courter. Had it not been for the Seles stabbing which greatly bolstered Sanchez's career, we would probably be saying this is a much more gigantic difference.
Seles 1991 U.S Open 35 year old Navratilova vs Serena 2002 U.S Open Venus. Edge Serena by ALOT.
Seles 1992 Australian Open Mary Joe Fernandez vs Serena 2003 Australian Open Venus. Edge Serena by a Pacific Ocean.
Seles 1992 French Open Graf vs Serena 2003 French Open Henin. Sort of even considering Graf vs Henin on the specific surface of clay, but Henin at the time was in way better form, especialy on clay, than Graf.
Seles 1992 U.S Open Sanchez Vicario vs Serena 2003 Wimbledon Venus. Edge Seren by ALOT.
Dont ask me to get into semifinal and quarterfinal opponents as the comparision will only get alot worse for Seles.
As for Navratilova she was 35-37 years old at that point, so dont make me laugh. Venus, Henin, Hingis, Davenport, Clijsters, Mauresmo, pretty much everyone in the top 10 when Serena completed her Serena Slam over the deepest womens field ever >>> 36 year old Navratilova. That Navratilova at that age was still the 3rd best player in the World speaks volumes to the Graf/Seles era which ****s of those two players try to build up into something it never was. Yes Serena dominating at 31 speaks volumes to todays era too, but nobody ever suggested right now is a strong group of players, but she already proved herself vs ones in the past.
As for injuries, Serena has Seles beat in bad luck by injuries by a country mile. Serena sustained two huge injuries at the height her dominance. Her knee surgery after winning 5 of 6 slams (and would have been the heavy favorite to win the next 2 on hard courts which were won by Henin, who never beat Serena on a non clay court until 2007), and then her restaurant accident which led to heart problems, infections, which even had her in danger of losing her life at one pont. What Seles had as far as bad luck worst of all of course was not injuries but the terrible stabbing, which is the worst thing that happened to either player I concede, but apart from that any further injuries which you seem to be focusing alot on (note you brought this part up to begin with, not I) in Seles's 96 onward career, Serena had more and bigger ones, but simply dealt with them better, and her game stood the test of time better. The next generation of players did not bypass her former standard of power, shotmaking, and athleticsm, the way they did Seles. Seles deserves credit of course for setting a high bar, but it was a bar others were able to jump over before the decade was out, while the bar Serena set a full decade ago is still the standard no others have approached, hence why she can continue so long and continue to make comebacks from adversity and win slams.
You say Serena couldnt deal with Henin in 2007. While this is mostly true, I fail completely to see your point. Seles could not deal with any of Hingis, Davenport, Venus, Serena, a now aging Steffi Graf, and even an aging Jana Novotna, every single year from 96-2003. Check out her records vs that entire group of players in this span. I know what will be said, well this wasnt prime Seles anymore. Well 2007 was not prime Serena. 2007 was probably Serena's 9th best year of tennis to date (behind 1999, 2001, 2002, 2003, 2008, 2009, 2010, and 2012), and Henin's best year of tennis. So good for Justine, she was better in her best year of tennis than Serena in her 9th best, I would sure hope a top 15 player of all time that would the case. Similar to how Seles in her best and second best years of tennis were better than Steffi Graf in her 9th and 10th in 1991 and 1992, again if that wasnt the case for the former then they must really not be that good period. Lets compare Seles in her 9th best year to another great player in their best and see how that went, actually we already got a bit of a taste of that with Seles in 2000 or 2001 vs Venus, or Seles in 1997 vs Hingis. Henin meanwhile in early 2008 was sent into retirement with a 6-2, 6-0 ass whooping to a resurgant Serena, and sent back into retirement by losing any momentum in her 2nd comeback with a defeat to an injured Serena who was wrapped up like a mummy in the 2010 Australian Open final.
Lastly you have a very incorrect fact. Seles never won 7 of 8 majors or 8 of 9. This is already purely impossible when she never won Wimbledon her entire career. She won 6 of 8 majors in 1991 and 1992. Serena at her peak won the Non Calendar Grand Slam, while Seles could not even manage a Career Slam.